In the chaotic world that is Iron to Silver, you have no one to rely on but yourself. As a support, often you can only hope to have a decent adc to actually support. A good support can make a good adc better. However, a support can't really do much to help a support that can't even last hit properly. So to remove that inconsistency, why not just pick up Senna, an adc disguised as a support.

In my opinion Senna has one of the most fun and well balanced kits in League. Easy to pick up and high skill ceiling.

Absolution - Senna's passive is what allows her to scale so well into the late game. Hitting a champion twice with an attack or ability allows her to collect a mist. Minons that die around gives a chance to drop mists which she can collect.

As a support this allows you to focus on picking up mists, and harassing your enemies.

Piercing Darkness - Senna's Q, aside from her AA is her main form of damage, sustain, and poke. Good positions allows her to heal an ally, poke an enemy, and maybe even collect a mist. Good skill.

Last Embrace - Main form of engage and peel. Compared to Lux and Morgana snare, Last Embrace has a much longer cooldown. Ideally, you save this skill when you get ganked, or when you can set up a kill.

Curse of the Black Mist - A light escape tool. Gives you MS and camo. This is usually used to run away after teamfights. Alternatively, you can use this to gank an enemy while hiding multiple teammates in your camo. That is if your teammates know how to utilize it.

Dawning Shadow - Global ultimate are always good. Plus this skill can be used to save your dying teammates, snipe dying enemies or even both. Pretty much a large version of your Q. Good ultimate.

Primary Runes

Inspiration is the rune path that you will use 80% of the time.

The slow from Glacial Augment is just too good to pass up. The slow allows you to land your Piercing Darkness with ease. During early game, this makes harassing a lot easier.

Magical Footwear is pretty much the best of the 3 choices. You wont really be using the stopwatch to build into anything or using hexflash.

Biscuit Delivery gives you better sustain in lane.

Approach Velocity Synergizes well with glacial augment. As an added bonus it also synergizes with Last Embrace

Inspiration synergizes pretty well with Senna's poke and run kind of playstyle. It also provides a lot of utility. Honestly, I can end the guide right here since Inspiration is good enough. But, I think there are a few other runes that can be used to switch up her play style.

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Domination would be a second choice.

Dark Harvest Would be the best among the four choices. Being able to gather mists and souls as the same time is a late-game dream. While Senna can be bursty when she is fed, it is not that practical to take Electerecute over Dark Harvest. Plus, dark harvest can help finish off low health enemies along with Dawning Shadow

Cheap Shot - Not really the best rune you can get since it requires you to land you W first.

Taste of Blood - Helps with sustain, doesn't need any pre-requisites to proc. The best of the 3

Sudden Impact - Requires your E to be active to proc.

Eyeball Collection - Its the low elo, no one really wards properly enough to warrant the use of the other 2 runes. What is not lacking in low elo though? Thats right, champion takedowns.

Domination is the rune path to take when you expect a game to last 40 minutes+. Its not as good as inspiration, but it can give some variety to Senna's gameplay. Dark harvest helps in executing low-health enemies and in long games, the little damage you use to execute can be game-changing.

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The third and last possible rune path you can take is Resolve

Resolve isn't a rune path you will take in many games. It is good in matchups where you are against many bruisers like Darius , Pantheon , or Riven.

The reason why you should take Resolve as the rune path is that you would want to increase your survivability in dueling.

The only rune you should even consider is Grasp of the Undying the other two are pointless. Sure guardian helps you support, but again you are a support carry not a traditional support. Aftershock is useless since it requires you to land your E and get close to your enemies, which is not the best way to play Senna.

Mix and match to your whatever you like for your second and third rune.

For your fourth rune Revitalize makes your Q heal and ultimate shield a lot better.

Again resolve is not the best rune path for Senna to take but yes it is viable. Especially if you pair it with Immortal Shieldbow and Black Cleaver

One of the reasons why Senna is such a fun hero to play is because of her flexibility. There are 2 main builds with Senna. The first is to build crit the second is lethality.

While Senna is a scaling champ, she definitely shines in early game. Except for her level 5 threats, Senna can outzone and outpoke most supports. Her absolution provides a surprisingly good burst when harassing enemies. If the support of the enemy is passive, then just go for the adc.

Your early game trades are almost always one sided. All you have to do is walk to the enemy and use your combo. Autoattack + Q. In one combo, you have hit your enemy twice, proceed absolution and healed yourself. Meanwhile your enemy would have hit you with 1 autoattack.

You must harass in the early game. Only play passively if you are not warded. Keep your enemies health low so that it would be easier to set up kills when you land a W or if your jungle ganks.

Playing aggressively allows your adc to have some room to farm freely while denying your enemy cs.

To summarize the early game. Play as aggressively as you can without dying.

Mid game depends on how well your team is doing.

Is your adc doing well?
No - Then stay with your adc
Yes - Roam

Is your team doing well?
No - Roam
Yes - Stay with your adc.

Is your team and adc doing well?
Yes - Good for you
No - Roam and farm until late game.

In the midgame, if you have been able to get a few kills and a few items, it is possible for you to snowball despite not being in the late game. If so, it is better to win the game early, than to lose the game late.

Your trinket should have transformed by now so you should sell your ward. Make sure to keep the dragon side warded. If needed, call for DR and objectives.

At this point, you would be outranging everyone. Your job is to be the second adc. Take advantage of your range and keep to the backline. Even with your high range and high damage, do not go solo.

The late game is pretty much very straight forward. Stick with the front lines, keep to the back, do not die.