Hello, I've been playing LoL sense beta and decided to make a guide on a underrated champion: Ap corki mid. I am currently 2200 elo (diamond 5) and I thought I might share something with you guys from my experience.
I started playing ap corki mid lately and found to have great sucsses with it.
People underestimate his dmg and will often prefer to focus other targets.
So here is my guide and I will explain why i choose to play him as I play him.
Everything I will explain here is my opinion on how to play this champion, I'm not forcing you to play him like this, this is how i feel how you should play it.
This is my first guide so please don't hate on me :P
P.S Sorry if my English is not the best. :)

Ok, so first ill explain why i start Doran's Blade. Corki as we all know is a ranged champion and has great escape skills, so I prefer to start with the Doran's Blade because it gives you good sustain with the hp and health regen it gives each hit. It also helps with cs because of the 10 attack dmg. Early game corki mostly needs to focus on farm because till 6 hes kinda weak, so i prefer this item as i find it to be the best.
After your first back you will want to get the Tear of the Goddess to clear out your mana problems and corki AP is a poke champion, which you will want to spam your skills so the Tear of the Goddess is a must.
Sheen is a great after item after help you finish off enemys.
Rabadon's Deathcap, Lich Bane and Seraph's Embrace are items you will buy most of your games.
Rabadon's Deathcap is to fullfill your poke dmg, Lich Bane is great with the movement speed buff and to finish off enemeys and Seraph's Embrace will complete your tear.
By the time you reach the Seraph's Embrace your tear will be or almost be full because corki likes to spam his skills. The active on this item will also make it harder for the other team to kill you. (As it wasnt hard enough)
All the other items mentioned really depend on who your facing and how the game is going-
If I'm facing a zed mid I would get Zhonya's Hourglass to deny his ulti. Or If they have full ad team I would also get it, It really depends on the game.
Enemy team Stacking MR? - get Void Staff
Getting foucsed? - Banshee's Veil or Guardian Angel
* Mejai's Soulstealer - This is a risky item but, If you feel like your fed enough and can go on without dieing. Why not? Go for it. It's a great item because corki will mostly poke in the back in teamfights and people forget to focus him. * I would only get this item after i got atleast a Rabadon's Deathcap and a Lich Bane.

Ok so your main question is going to be-
Why is he maxing Gatling Gun before Valkyrie
The answer is simple, although Valkyrie scales on ap and Gatling Gun doesnt,
Corki will use his Valkyrie for 2 situations :
1. Disengage/escape 2. Chase for a kill
Yes, you can Valkyrie over your enemy for the ap dmg it does, But Most of the time its just not worth the risk. Thats why I prefer to Max Gatling Gun before Valkyrie.

I choose to take Barrier and Flash.
Flash is a must no need to explain..
So why Barrier?
Corki Is a Ranged Poke Champion that will want to poke his enemy from a far range and will not always get into close range combat (unless hes chasing a low health champion). Barrier will also help you with early game ganks and overall harder to kill. Ignite can only mostly help you early game and even with it you will probably need the junglers help to get off a kill.
Teleport is situational as you might want to take this against a roaming champion like twisted fate to try to counter his ganks.

I just find Barrier to be the most useful.

Runes are pretty state forward, I take flat armor seals yellow because you will be less vulnerable to auto attacks and just always good to have some extra armor.
For gylphs I took the abilty per lvl to max out his mid game dmg and help him with late game a bit.

Other Runes To Consider

Greater Mark of Hybrid Penetration Greater Mark of Hybrid Penetration

This is a great rune combined with Greater Quintessence of Attack Damage if you think you can bully your enemy and zone him out of creeps. This will mostly work Vs melee champions who are weak with trades pre 6 like akali.


Greater Seal of Scaling Ability Power

Get this if you wanna go full glass cannon.


Greater Seal of Scaling Health

If attack dmg isn't such a threat in the game then you can take this.


Greater Glyph of Magic Resist

If you feel you need some Mr for the game then go ahead.


Greater Quintessence of Attack Damage

Mentioned above

The more farm you have the more items you'll get the stronger you'll be. (duh)
Everytime you can, Go farm off those wraiths it will help you more than you think. If you can farm your enemy's wraiths also to extend that gold lead as much as possible.

So I said before the early game you should mostly farm and try to stay alive because untill you reach lvl 6 you dont have really enough dmg to burst your enemy rather poke him with Phosphorus Bomb each time you can do it safely.
The mid game is where corki shines, when teamfights are heading, try to poke your enemy with Missile Barrage before the teamfight even starts.
Once a teamfight has started you should burst down your enemy with Missile Barrage and if the enemy is low enough jump in with Valkyrie and finish him with Phosphorus Bomb.
Ofcourse you need to know how much Dmg you can pull out so just jump randomly 1v5.

So that's about it, kinda short but I think I focused on the main things you need to know.
Again this is only my play style of how I play ap corki and not everyone has to agree with me.
Hope I helped some of you summoners out there and I would love to get feedbacks on what you think of it.