Feel free to comment, criticize ( every useful comment, even negative ones, will be taken into account ), and correct me if my english is wrong : it is not my native language, and I'll try to correct any mistake I could've made ASAP.
Thx for feedbacks, any remark is appreciated !

I MIGHT NOT BE ABLE TO DO REGULAR UPDATES UNTIL ~ 2ND WEEK OF AUGUST 2017
Incoming, in this order :

• Builds, and build order
• Laning Phase
• Midgame role and fights
• Lategame role and fights
• Tips, tricks for advanced players : some weird powerspikes, all-in strategies, mechanical moves ...
  
  If there is anything else you'd like to see in such a guide, let me know : for example, I don't plan to include videos but I could if this seems important to a lot of people.

Hey ! This guide tries to give to as many players as possible the capacity to understand Cassio builds and kit, to become familiar with her gameplay, to know her strengths and weaknesses, to play with her, to play in her team or in the ennemy team ...
To sum up up, I will try to be as precise and complete as possible, for beginners aswell as confirmed Cassio players who might want to try some new things with the Snake !

I'll present briefly Cassiopeia :
Cassiopeia is a mid-range AP champion. She has a huge lategame, and a weak early game. She is mainly used for her single target DPS, which is one of the strongest in the whole game.
Due to her unique passive and kit, she is a very interesting champion to play, and requires good mechanical abilities.
She is really versatile and can be useful in very different situations, with and against many different team compositions or matchups.

To understand each point of this description, we'll focus on Cassiopeia's spells, to determine what builds, runes or masteries are the most interesting for her to run, what could be her strengths and weaknesses and why, how to deal with them, and how to finally make the best use of this beautiful champion !

Let's focus on Cassiopeia's spells to understand her better :

Passive

Serpentine Grace (P)
This passive is really, really powerful. Even if it might look underpowered at first sight, it grants Cassiopeia movespeed, making her being able not to buy boots, and so being able to purchase a sixth combat-item. Her movespeed is slightly higher than what other champs get with upgraded boots ( ~ 380 for most champs ) at level 18, and becomes quickly effective.
The capacity to build 6 items without having to buy boots is one of the reason which gives Cassiopeia a huge lategame power.

Noxious Blast

Noxious Blast (Q)
Noxious Blast is an AoE delayed ( ~0.3 sec ) skillshot, with several effets :

• It deals a few damages to ennemy hit : it is a DoT, lasting for 3 second.
• It grants Cassiopeia movespeed if one or more ennemy champions are hit
• It poisons the ennemy ( minion, monster, ennemy ... ) : the ennemy is poisoned for the DoT duration ( 3 sec ). The poison use will be detailed in the Twin Fang explanation.

Second spell to upgrade. It gives damages, in itself and combined to E, movespeed which is necessary to Cassiopeia, with a low CD, a good range, and a decent mana cost.

Miasma

Miasma (W)
This spell is unique thanks to his passive capacity ( even if it is similar to Poppy's W ).
It is an AoE ranges skillshot, with a minimal range ( ~300 ). It poisons the ground in an arc of a circle, with clouds lasting for 5 sec. Ennemies caught in the clouds take damages as long as the stay in them, are poisoned, and can't use any movement abilities. Like the Q, it has differents effects :

• First, it deals damages. However, this is not a spell you'll use to damage your ennemy, and unless they stay in it for seconds, this won't make a huge difference.
• But it also poisons ennemy : if you miss a Q spell, which can be easily dodged, you can use Miasma to poison you ennemy and be able to DPS them with Twin Fang ( cf next paragraph ).
• Finally : As long as an ennemy stay in the clouds, he cannot use ANY movement ability. Lee Sin's Q ( first and second activation ) for example, or any channelled movement capacity ( Zac's E) but also Fizz's E, Twisted Fate's R, Ekko's R, LeBlanc's W ( first and second activation ) are amongst the interesting one.

Multiple clouds are thrown with Miasma : the Yasuo's Wind Wall blocks any cloud going through it, but those who don't still remain on the ground.

Twin Fang

Twin Fang (E)
This spell is the first you'll upgrade and max, always. This is your main DPS tool.
Twin Fang is a targeted abilty, dealing damages to ennemies. Its damages are greatly increased if the target is poisoned.
The main strenght of this spell is its ridiculously low CD ( 0.75 sec, down to 0.45 with 40% CDR ), making Cassiopeia one of the ( if not 'the' ) highest single-target DPS in the game.
It also allow Cassiopeia to sustain, as it refunds her HP, increased when the ennemy is poisoned. Combined with a Rod of Ages, it gives Cassio a great survivability during squirmishes or even teamfights.
During the laning phase, it is an excellent tool to farm, as its mana cost is totally refunded if the target is killed by Twin Fang.

NB : Mana cost is refunded even if the target dies during the animation ( which is different from Irelia's Q, for example ).
Damages are increased on ennemies that are poisoned by an ally ( Teemo's "shrooms", Singed's Q ... ), making it easier for Cassio when played with one of these champions.

Petrifying Gaze

Petrifying Gaze (R)
Another unique abilty in the Cassiopeia's kit. This is the only hard CC she has, but it is a very powerful one : Petrifying Gaze is a non-targeted ability, with a short casting time. In a cone in front of you, ennemies are damaged, slowed, and those who are facing you when the casting time ends are stunned for 2 sec.
As it has a short casting time, ennemies may react to the sound + animation and turn away from the Cassiopeia. However, it also allows Cassio to Flash during the animation. If so :
The Cassiopeia's orientation do not change with the flash, no matter the direction she chose. The ultimate will be "launched" from the point she flashed to, allowing her to Petrifying Gaze + Flash to suprise ennemies that are far from Cassio, or to stun someone fleeing in front on her by casting her R behind her, and then flashing in front of the ennemy.

Globally :

You will always start with Twin Fang (E), since the rework in S6, and the E not needing to be land on a poisonned target to have a low CD, there is no interest in starting with Q.
You will always take Noxious Blast (Q) level 2, as it gives you some push potential, and a good trading capacity. Theoretically, if you empty your Mana pool with Q/EEE/Q/EEE [...] you'll outtrade every single champ in the game.
The only debatable point is whether you should take a second point in your E, or take your third spell (W) when you up level 3. Here is what I suggest :

• If you play against someone whose champion doesn't have any movement ability ( Syndra, Annie, Lux ... ), and the ennemy jungler doesn't have one either, or if you're certain that he won't gank you, and you want to have the biggest trade potential to all-in : take a second point in Twin Fang.
• However, if you think that you might get ganked, or if the ennemy is a Fizz, a Zed, or anyone with an all-in potential with a dash at level 3, take your W. The slow will allow you to survive more easily to any gank, and the disable of movement abilities makes you safer against any all-in.

After this, upgrade your spells following the priority order given earlier. Taking a third point in Q before your lvl 9 is not advisable, as it will delay your first powerspikes, and the damage + movespeed gain won't make a huge difference compared to your E max.

With these abilities, we see that Cassiopeia is designed to be a high sustained-damage champion. To help her "DPS-ing" during a long time, her passive Serpentine Grace and Q Noxious Blast give her movespeed, allowing her to kite pretty well both offesively and defensively, and the stun granted by her ultimate Petrifying Gaze allow her to DPS freely for 2 sec and to reposition herself in a better place to keep the fight going in midrange.

This gives us a direction to give to our builds and Cassiopeia will need :

• Amplifying Tome AP and magic penetration, obviously, as her job is to deal damages. But also
• Sapphire Crystal Mana, to be able to make fights lasting long; the Twin Fang mana cost, despite being low, shreds Cassio's mana pool as it burns 160 Mana pts per second with ~40% CDR.
• Fiendish Codex CDR, to maximize her damages. This point will be explained more precisely later.
• Ruby Crystal HP, and more generally defensive statistics, to survive to burst and outdamage her opponents during longer trades.

CDR is VERY effective with Cassiopeia. Despite having pretty low base CD ( 0.7 sec on Twin Fang, 3.5 on Noxious Blast ), Cassiopeia benefits from CDR more than many other champions :

• First, as she relies a lot on her (Q) to poison the ennemy and deal damages, and as it is a skillshot, missing a Q without CDR means that Cassio's damage output will be extremely low for 3.5 sec, while this duration is reduced to 2.1 sec with 40% CDR. The time gained, ~1.5 s, is enough to cast three Twin Fang, which is HUGE
• But it also multiplies the Cassiopeia's DPS by almost 1.7
  
  Imagine what could a LeBlanc with 40% CDR do in a fight compared to a 0% CDR one. They'll both use Distortion at the same time, then Shatter Orb, etc. But, when the first LeBlanc will recover her Distortion cooldown, she might not be able to use it immediately. She has the potential to (W) faster than the other LB, but she might have to wait to use it in the best way. Thus, the 40% CDR gives a lot of potential to LeBlanc, but her damage output, in certain case, might be the same with or without CDR.
  This explanation goes for many champions : the idea is that sometimes, you have to wait before using an ability, to make the best use of it.
  
  For Cassiopeia, the situation is different. Indeed, she can, and she will, spam Twin Fang whenever it is available, as you don't have a "best way of using it" or any requirement : if you are in range and you want to DPS, you can use it. So your DPS phases in fights will be multiplied exactly by your CDR : the formula is simple, as having X% CDR means that your CD are reduced by X%, thus making you able to cast them 100/X times more in the same amount of time. If you have 40% CDR, you use your spells 100/40 = 1.66 times more, making your Damages Per Second multiplied by almost 1.7 with Cassio as you effectively use your spells as soon as they are available.

You see that CDR, for a Cassiopeia, is a fundamental statistic : not only it gives her access to the (Q) movespeed, (W) zoning or (R) CC more often, it also multiplies her DPS more than any other stat. Thus, in our runes and build, we will try to get 40% or at least 30% CDR without the blue buff.

We can know determine precisely what are the strengths and the weaknesses of Cassiopeia :

Pros + Ranged champion + Huge scaling + Unique gameplay and kit + Gamebreaker ultimate + One of the highest single-taget DPS + Very interesting to play, outplay potential + Can destroy a game alone

Cons - Squishy without defensive items - Low escape - Relies on good mechanics, especially kiting - Weak early game - Has some very hard counters - Based on skillshots landing

We have already presented what statistics are important as a Cassiopeia player, what strengths we can rely on to win, and what weaknesses we should try to compensate, within runes, masteries and build.

Standard runepage

3

9

6

3

9

• Greater Quintessence of Ability Power * 3 These are the standard quintessence you'll run with an AP midlaner, nothing really special there.
• Greater Mark of Magic Penetration * 9 These runes allow you to deal damages in early game, compensate the MR runes of your ennemy, and will synergize later with Void Staff or Liandry's Torment.
• Greater Glyph of Scaling Cooldown Reduction * 6 Having 6 of them grants you 10% CDR at lvl 18, and as I explained in a previous point, CDR is one of the best offensive statistic for Cassiopeia. Combined with a Morellonomicon and another 10%CDR item, or the blue buff, you'll reach 40% CDR.
• Greater Glyph of Magic Resist * 3 The goal is there to reduce the early damages of your opponent, who is most likely to be AP in the midlane, and later on, any upcoming magic damage from jungler, support or toplaner.
• Greater Seal of Scaling Health * 9 These runes become very effective as the game goes on, are way more useful than Armor Seals against an magic damage-dealer opponent, and allow Cassiopeia to survive the burst she will have no way to avoid due to her low escape capacity.

This is the standard rune page you will run against a magic damage dealer opponent, who is not supposed to destroy your early game, and when you don't want to scale on CDR without Morellonomicon. Some players run Greater Quintessence of Movement Speed, but I tend to think that Ghost, {{Stormraider's Surge]] or even the passive and the (Q) are enough to dodge skillshots and roam, or kite.

Against a physical opponent

• You MUST take Greater Seal of Armor * 9 instead of the HP Scaling one. Cassiopeia base armor is quite low, these opponents ( Talon, Zed ... ) often run Lethality runes and tend to build physical penetration-based items. They also are assassins, or at least good duelist, and will try to kill you. Running HP against them will be inefficient, as their penetration will allow them to deal true damages to you and destroy you.
• You might want to change your three Greater Glyph of Magic Resist to Greater Glyph of Scaling Magic Resist, or Greater Glyph of Ability Power depending on the magic damage you will be facing later on the game. This is not fundamental, as they will always be some source of magic damage in their team, and you only have 3 runes of this type anyway.

Against lategame team composition

In this case, the point is that you won't be useful early on no matter what, as a Cassiopeia. One idea could be to run flat runes to try to snowball quickly, but another one is that your lategame will be hard so you have to be as powerful as possible yourself. In other words, you let your team decide what the early/midgame will look like, and you will try to carry them later.
In this situation, you might want to skip the Morellonomicon in order to get more offensive items ( Void Staff ... ) and some real defense ( Banshee's Veil, Zhonya's Hourglass ... ). By doing this, you lose 20% CDR without the Morello, and need to compensate this loss in your runes.

I suggest you to run :

• Greater Quintessence of Scaling Cooldown Reduction * 1 along two flat AP runes
• Greater Glyph of Scaling Cooldown Reduction * 9

This allow you to have +20% CDR at lvl 18, which can become a 30% or 40% with one or two other items, or the blue buff.

This page is quite specific, and gives you a terribly weak early game. However, you become a scaling monstruous DPS machine, with a massive AP/Magic pen build where CDR isn't a stat you need to invest on.

Now, I'll try to explain and justify in detail each spending of mastery points, and more generally, to explain why and when you have to play with one keystone or another.

I will first compare every step of the two first trees, and the 4 first ones of the last tree, in order to give a precise explanation. I will then give a few examples of pages, refering to their keystone, rather than presenting every point for every set.

The little conclusion after each mastery is not a "must-follow rule", but only what the usual choice would be !
All the explainations are made in the paragraphs, so read them if you want to understand when and why these choices are the good ones !

• Fury vs Sorcery : Nothing special to say there, as you are obviously scaling on AP more than AS :
  5 points in Sorcery
  
• Fresh Blood vs Feast vs Expose Weakness : The real debate does not involve Expose Weakness , as you are the main damage dealer of your team, and you mainly deal damages to one target at a time.
  I personnally prefer Feast, as it gives a very interesting sustain during the laning phase, and combined to your Twin Fang sustain, you'll be able to farm safely for hours.
  However, in some very easy matchups ( Ryze, Karthus ... ), or some MU where you are seeing for pre-6 pressure ( typically vs all-in melee champ, where sustain won't change that much : Akali, Diana, Yasuo ... ), Fresh Blood can be a very good option :
  1 point in Fresh Blood or Feast, depending on the matchup.
  
• Vampirism vs Natural Talent : Both choices are fine. I prefer to spend 5 points in Vampirism , that gives spellvamp which gives a huge sustain in lategame fights, combined with Rod of Ages + Twin Fang.
  Natural Talent is useful only for its AP part, and you are already reaching enormous AP amounts, so I do not think it is necessary :
  5 points in Vampirism .
  
• Bounty Hunter vs Double Edged Sword vs Battle Trance : I always use Battle Trance , because as you are not looking for an instant burst, you'll end up quickly fully using its bonus, even in early game trades.
  Meanwhile, Bounty Hunter requires you to kill a lot of different ennemies to be effective. As Cassiopeia do not roam that much, it is not really worth the price.
  Finally, Double Edged Sword only makes you more vulnerable to early kills, for only a few percent more damages than Battle Trance :
  1 point in Battle Trance
  
• Battering Blows vs Piercing Thoughts : Nothing interesting here, as you are doind almost 100% magical damages, even with Liandry's passive or Luden's one :
  5 points in Piercing Thoughts
  
• Warlord's Bloodlust vs Fervor of Battle vs Deathfire Touch : Even if there is no real choice there, as Deathfire Touch is the only one interesting for us, I'll explain why it is a good mastery :
  It is the keystone which will bring you the most damages in lategame. Moreover, with Cassio, your DoT ( Noxious Blast, Miasma ) and your AP amounts thanks to your 6 items will make it even more effective. It will be refreshed constantly with your E spamming, thus making it a permanent DPS buff. Combined with Liandry's Torment, your poisons will end up doing almost 1.5 times more damages than what they should :
  1 point in Deathfire Touch

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• Wanderer vs Savagery : As you will get stuck onto your midlane if your opponent decided to roam faster than you ( for example, Ahri, LeBlanc ... ), you will need extra movespeed to join the incoming fight, which can be a terrible 3v2 crush if you cannot manage to be there in time. Plus, the extra damages given by Savagery are not really necessary, as you will last hit pretty easily with Twin Fang :
  5 points in Wanderer
  
• Runic Affinity vs Secret Stash vs Assassin : you won't benefit much from Runic Affinity as you will probably wait until your first blue buff, and you will never take the red one. Thus, this is not a good investment.
  Secret Stash can be good, but you will need to buy some potions to make it efficient, and this is not likely to happens as your innate sustain is enough in easy lanes, and Corrupting Potion is there in hard ones.
  Meanwhile, you will stay in midlane for a while, and so will be playing in duel :
  1 point in Assassin
  
• Merciless vs Meditation : Both masteries are useful to play. As it depends on your playstyle more than the other masteries ( afk farming or seeking for kills ), I do not recommend anything special as long as you play according to the point you chose ( farming with Meditation , trading with Merciless ). Another viable option is to split the 5 points you get, and this is what most people do :
  3 points in Merciless , 2 point in Meditation
  
• Greenfather's Gift vs Bandit vs Dangerous Game : You won't autoattack that much, and are a ranged champion, making Bandit almost useless.
  Greenfather's Gift might be good toplane, as many bushes are along the lane, bu it midlane you will almost never be able to use it.
  Dangerous Game allow you to sustain both mana and HP during fights, as you will probably get some assists or kills, or it might allow you to survive a DoT that would have killed you after your opponent died :
  1 point in Dangerous Game
  
• Precision vs Intelligence : Despite being extremely efficient on Cassiopeia, the extra-CDR given by Intelligence is not enough to justify its price.
  Indeed, the magic pen will scale extremely well with your build, as you will probably get a lot of AP before buying any magic pen. Moreover, it will give you a flat ~10 Magic Pen at level one, multiplying your damages more than 5% CDR.
  To be precise, 5% CDR will give you, ultimately ( lvl 18 + full stuff ) more damages than the 8.4 magic penetration, if your ennemies build Magic Resist. However, even at this point, the difference won't be much, and having a decent early game will be a lot easier than betting on super lategame fights :
  5 points in Precision
  
• Stormraider's Surge vs Thunderlord's Decree vs Windspeaker's Blessing : as you have no shield or heal towards allies, the third keystone is definitely useless.
  Stormraider's Surge is extremely powerful. You will have no problem using it, as you can (Q) + 3*(E) during the 2.5 sec needed to trigger it. The huge movespeed + slow reduction given by the keystone, combined to your Noxious Blast's MS, and sometimes Ghost, will make your ennemies go mad, as you will be running way too fast for them. However, it requires good mechanics, as it won't be easy to kite well ( not too close, not too far ) and to dodge skillshots. If you just move randomly at 600 MS, this is a bad use of the keystone !
  Thunderlord's Decree is also viable. In laning phase, you can proc it very easily with this combo:
  Noxious Blast -> Twin Fang + Autoattack
  This might help you to pressure easy lanes, with the same idea that I already explained in the first tree for Fresh Blood . However, once the game goes on, this little burst won't make a huge difference, and deathfire touch would be a better choice to deal damages later in the game :
  1 point in Stormraider's Surge
  

---

I will not present the last 2 lines, as you will never use them. You are a carry, with little-to-no crowd control if you miss your R, so you cannot afford the Resolve Tree's Keystones.

• Recovery vs Unyielding : You won't stack a lot of Magic Resist or Armor, besides your basic statistic or runes. Thus, you have no interest to play with Unyielding :
  5 points in Recovery
  
• Explorer vs Tough Skin vs Siegemaster : if you are playing with this tree, you are facing a ( very ) hard lane. Thus, you will try to use the defensive masteries of this tree.
  Siegemaster might look as good option, however you will usually have enough range not to be poked while under tower : the danger will come, precisely, while you are out of your tower's range.
  But Tough Skin will be useful for every autoattack trade you might take in early game, and above all, it will reduce the damages coming from the minions :
  1 point in Tough Skin
  
• Runic Armor vs Veteran's Scars : Both choice are viable. If you fear the early game, go for Veteran's Scars . The bonus sustain given by Runic Armor is not neglectable either, but you might look for earlier stats if you are spending that much in the Resolve Tree :
  5 points in Veteran's Scars
  
• Insight vs Perseverance vs Fearless : Despite Insight looking pretty good at first sight, having your flash 45 sec earlier won't always save you. Plus, it is not as teamplay oriented as a toplaner having its Teleport 45 sec before the ennemy one, and using it to make TP plays.
  Perseverance can also look great, but you won't have that much problem sustaining from low life. Moreover, you will not try to stay in lane vs lane-bullies with less than 25% life, thus making it hard to use effectively.
  Meanwhile, Fearless grants you huge capacity to survive bursts :
  1 point in Fearless

---

18/12/0 :

This is almost viable in any situation, as your primary role is to be an AP carry.
Deathfire Touch is the keystone that will grant you the maximum damage output during fights, if you manage to survive long enough to DPS !

This mastery set is more effective against tanky compositions, where your team already has peel for you ( you are playing with Shen, or Lulu, or other 'disengage' or peel champions ). It is also good when you don't try to take an early advantage by yourself, or to outplay your direct opponent, but when you are seeking for midgame or lategame teamfights, or when you follow your team.

To sum up, it is not that much a "soloQ" keystone, as it won't allow you to outplay your ennemies, and if you somehow manage to get fed, you might be hard focused while your team doesn't react - remember these games where you were 5/0/3 and your allied Shen decided to ALWAYS ult your feeding ADC instead of you.

12/18/0 :

This is my standard masteries page, and should be yours too. I already explained some of the advantages of the Stormraider's Surge in the dedicated paragraph, but I'll explain there which team composition makes it optimal :

You will make the best use of this keystone in games where most of the opponents are squishy ( so you can proc it easily - forget it against a Shen/Maokai/Galio/adc/Blitz draft ), and where you excpect yourself to outplay your ennemies : you rely on the extra movespeed, which is not a combat statistic in itself, to dodge/kite/reposition during fights.

This is also good against "diving bruisers" composition ( Irelia, Riven, Fiora, and more generally Diana, Akali ... ) where you will be focused even if you position well. Running Stormraider's Surge will allow to hit-and-run easily and to survive a violent bruiser chasing you.

To sum up : always a good choice, as it allows you to outplay people, can be used offesively or defensively, and does not rely on teammates or items to be effective. However, it needs a bit of training to be used, as it might surprise when it triggers, and requires a bit of "mechanics".

12/18/0 :

Not a keystone I recommend, however it can be useful in some very specific situations : if you clearly have an advantage over your ennemy midlaner, and if you don't think you'll need the extra damages from Deathfire Touch or the extra movespeed granted by Stormraider's Surge , you can run it in order to destroy quickly your opponent, to snowball.

However, as Cassiopeia is not that good in lane-bullying, you might not be able to snowball hard and end the game alone, so you may think that taking DFT would have been better to DPS.

To sum up, it can use to snowball a game quickly, not playing according to your usual playstyle as a Cassio or to the team composition as a whole.

18/0/12 or 0/18/12 : Resolve Tree investment

The choice between the three keystones is up to you, according to my previous explanations.
However, the missing 12 points won't be invested in the Ferocity or Cunning Tree, as they would have been ususlly, for ONE reason : you are terrified by your direct opponent burst ( aka Syndra ), or by some super-hard engage + burst composition ( for example, Malphite AND Alistar AND an assassin ).

However, as hard engage composition are usually tanky, the damages and sustain given by the first two trees is not neglectable, thus reducing the 12 points in Resolve Tree only necessay in one case :

The BURST. As I already explained in this guide, this is the first threat you are facing as Cassiopeia. Then, you will invest 12 points almost only to have access to Fearless .

To sum up : use this page when facing Syndra.

Cassiopeia, due to her kit making her a good duelist as well as an sngle-target DPS champion, is able to run many different summoner spells. Each of them should be taken regarding your direct opponent in the midlane, as well as the entire draft on each side.

FLASH

Flash is a must-have summoner spell for Cassiopeia. Not only allowing you to lane safely and preventing you from dying during a gank, or a bad trade, it gives you a way to reposition yourself during fights as you have no dash/blink in your spells. It will keep you alive if you react quickly enough after an assassin all-in, or when a bruiser tries to dive the backline of your team.
It also gives her the capacity to engage through your Flash + Petrifying Gaze combo. This technic, as a move requiring a bit of practice to be used properly, will be detailed in a following section dedicated to this kind of micro-moves.

---

The second summoner spell

Here, many choices are up to you :


GHOST

Ghost will be your main summoner spell after Flash. It combines very well with the Cassio's natural ability to kite, which still can be a bit limited to survive to an Irelia, or a Riven who decided to kill you. With your spells having a good range - Miasma even having a minimal range - you will be able to maintain a ~500 to ~700 units between you and the opponent you are hitting, even if he tries to run away or to chase you.
During fights, you should be able to dodge most of the incoming skillshots that might hurt you or CC you, with Ghost, your passive, and easy-to-land (Q) in a 5v5 situation.
During laning phase, it is quite a weak summoner though, not giving any combat power comparable to Ignite or Exhaust. Nevertheless, you can try to roam way better than what you could without Ghost, to apply pressure to a sidelane. After having enough items, you could even 1v2 the ennemy botlane, during a weird roaming timing where no one would except you to go alone to the botside of the map.



EXHAUST

Exhaust is a good summoner to run, and can be used in almost every situation. However, the Ghost might be sometimes more useful, but your Exhaust will never be a "bad choice".
Indeed, this spell has two main effects :
First, it reduces the physical and magical damages dealt by the ennemy you exhausted for a brief time, this effect being instantly effecive as you cast Exhaust. Thus, as you will outdamage almost anyone during a long trade, running this summoner allow you to survive to any initial burst and so winning the duel, as you will sustain and DPS more than your ennemy once he has burnt his abilities. You can also turn easily a 1v2 situation, using your ultimate Petrifying Gaze on at least one of them, and exhausting the one that might not have been stunned. You can then kill one or the other, as they will have no real counterplay.

Secondly, Exhaust slows your target. This cool side-effect will allow you to reposition yourself if you somehow get surprised by a dash-in or a flash-in, or to keep an eenemy in range for your Twin Fang. Above all, it gives you a free Noxious Blast on the targeted ennemy, if he has no dash, as he will run too slowly to dodge it.

Matchups where this is a must-have :
Akali, Katarina, Yasuo, Zed

Matchups where this is a good choice nonetheless :
Ekko, Jayce, Kassadin, LeBlanc, Syndra, Talon ...



CLEANSE

Cleanse, for the same reasons, is a pretty good summoner spell.
As you are mostly vulnerable to burst damages, you absolutely don't want to be hit by a CC during which you could easily dive, thus not doing your long-term damage dealer role.
The Cleanse will allow you to escape a painful CC you were hit by, and to reposition quickly with the movespeed buff.
However, if you are too slow to react, or if you are already caught in a bad place, you will also need your Flash or at least your ultimate to escape or to win the fight, as you have no inner dash to reposition yourself after having cleansed the CC.

Matchups where this is a must-have :
Galio, Morgana, Lissandra, Twisted Fate

Matchups where is a good choice nonetheless :
Ahri, Anivia, LeBlanc, Ryze, Syndra, Veigar



Barrier

Barrier is not as powerful as exhaust to avoid burst damages, but it doesn't need to be cast on your ennemy, thus allowing you to survive to long distance damages. You will run it if you don't need ghost, and if you think the only real theats will come from long-range champions, or to untargetable champions. More specifically, it can be good against True Damages.
There is not any matchup where you must run it, but some are pretty easier with this instead of Exhaust or Ghost

Matchups where you can go for a Barrier:
Fizz, Lux, Orianna, Talon, Vel'Koz, Xerath

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I would not recommend to play with a Teleport, even when playing a hard lane, because you won't be able to teleport behind a fight, to flank or destroy the backline : you are too squishy and too easy to be focused.

I would not recommend to use an Ignite, as you don't need extra damages to kill : even in 1v1 situation, Exhaust is better as it gives you a free (Q) and thus extra damages with your Twin Fang spam. May be, against a Vladimir or a Swain, but even there, I prefer to go for a Ghost and rush a Morellonomicon or ask my ADC to do a Mortal Reminder.

Heal can be used as a substitute to Barrier. As you are more or less subject to a lot of Morellonomicon users in the midlane, or some champions like Katarina reducing your heals, I do not recommend it either.