15/05/2012 - Replaced Rylais with Morellos for the CDR and added Situational Items and added icons because i finally know how to make them :D

Some abbrieviations:
AD - Attack Damage
AP - Ablity Power
AS - Attack Speed
CDR - Cooldown Reduction
HP - Hit Points
MP - Mana Points

Why AP Yi? Because his early game harrass and late game burst damage is INSANE!
I made a poem on yi. :D
Yi is pretty boss;
Somone you cannot double-cross,
He is someone you wont like;
Because of his ALPHA STRIKE!!!
HaHa!
Anyways i spent alot of effort on this guide so pls no hate comments i would like constructive criticism tyvm. :D

Pros
Early game your Alpha Strike does nearly half the enemy mid lane's hp!
Your Meditate is great sustain!
Highlander refreshes Alpha Strike every time you kill someone making pentakills easy!
Late game your Alpha Strike and Lich Bane can kill a squishy!
Enemy's always underestimate the power of AP Yi!
Hes extrememely fun :D
Cons
CC completely owns him.
Really squishy.
Hard to do well if not fed early game.
Long cooldowns.

Summoner's Wrath and Summoner's Insight are for Ignite and Flash. I get all the AP I can get in the Offense tree. In the Utility tree, I get Meditation for mana regen and Runic Affinity for longer buff durations. If you are under level 30 then start with the Offense tree then going into the Utility tree.

For runes I get Greater Mark of Magic Penetration for more damage Greater Glyph of Scaling Ability Power and Greater Quintessence of Ability Power for more damage. And Greater Seal of Scaling Mana Regeneration for a bit more sustain in lane.

Double Strike Master Yi strikes twice every 7th basic attack.

Master Yi leaps across the battlefield striking up to 4 enemies, dealing magic damage to each enemy with a chance to deal 400 bonus magic damage to minions.
Cost: 60 / 70 / 80 / 90 / 100 mana
Cooldown: 18 / 16 / 14 / 12 / 10 seconds
Magic Damage: 100 / 150 / 200 / 250 / 300 (+1.0 per ability power)
Bonus Damage Chance: 20 / 30 / 40 / 50 / 60%
Your main source of damage, its crazy powerful with a 1:1 AP scaling.
Master Yi channels, gaining massive health regeneration, armor, and magic resistance for 5 seconds. Master Yi is also healed at the beginning and end of the channel.
Cooldown: 35 seconds
Cost: 70 / 80 / 90 / 100 / 110 mana
Bonus Health Regeneration: 128.33 / 256.67 / 385 / 513.33 / 641.67 (+3.67 per ability power)
Beginning Heal: 12 / 23 / 35 / 47 / 58 (+0.33 per ability power)
Ending Heal: 25 / 50 / 85 / 125 / 183
Maximum Heal: 165.33 / 329.67 / 505 / 685.33 / 882.67 (+4.0 per ability power)
Armor & Magic Resistance: 100 / 150 / 200 / 250 / 300
Your sustain in lane and saviour in fights, this op meditate is just soooooooooooo good 4:1 AP scaling and more armor and mr? SURE BRO
Passive: Increases attack damage for as long as Wuju Style is not on cooldown. Active: Master Yi can activate this ability to receive bonus attack damage equal to double the passive bonus for 10 seconds.
Cost: 40 mana
Cooldown: 25 seconds
Bonus Attack Damage: 15 / 20 / 25 / 30 / 35
Doesn't do much but you can use it to proc Lich Bane
Increases Master Yi's movement speed by 40% and attack speed for a few seconds, as well as making him immune to any slow effects for the duration. Additionally, killing a champion during the duration will refresh all of Master Yi's ability cooldowns, while assists will refresh them by half the base amount.
Cost: 100 mana
Cooldown: 75 seconds
Attack Speed: 40 / 60 / 80%
Duration: 8 / 10 / 12 seconds
Your SUPERMAN ability refreshes Alpha Strike and Meditate when you get a kill! So gooood.

You want to get Alpha Strike up to rank 5 ASAP because it is your main damage ability. The order of skills is Highlander > Alpha Strike > Meditate > Wuju Style getting a point on Wuju Style at level 4 for a bit more AD.

Good
- useful to finish off kills
- for escaping and initiating
Okay
- to get out of cc if they have a cc heavy team
- synergises well with highlander
- if your facing ad in mid
Bad
- you dont need it, leave it for your top laner
- you have Meditate
- you have enough mana regen
- leave this to your support
- useless
- you wont die... EVER
- no comment

Mobility, Magic Penetration, its awesome :)
Adds to your already insane burst
Best AP item in the game

You have 3 starting choices:
1.
2.
3.
Personally I like to start off with Amplifying Tome and Health Potion becuase you can get a kage's lucky pick pretty early on in the game for that extra gold and it is the item with the highest ap that you can start off with.
However, if you are vsing an Ahri or Swain etc. you should start off with Boots and 3 Health Potion.
Mid game you should have completed your core items. This is when you will dominate, Deathfire Grasp their AP/AD carry and Alpha Strike will kill them instantly giving your team the advantage.
Late game when you have finished your build, you are very squishy so hide behind your team until one target is low enough for an alpha strike to kill. This is when you will pounce. Alpha Strike the team with Highlander activated killing your target and halving everyone elses health, Deathfire Grasp the one that was not hit by your Alpha Strike. Alpha Strike again and Lich Bane the tank giving you and insta-penta! Make sure to time this correctly and let them use their cc first as any stun will kill you.

Good AP, AS and CDR.
Against CC or AP.
Against Tryndamere Olaf or any other hard hitting bastard.
Against a heavy AD team.

Ahri 3/10 - Not that hard, just get Boots and dodge her abilities and your good to go.

Annie 8/10 - HARD ONE you want to play really defensive against her because of her ANNOYING stuns and insane burst giving you no time to meditate.

Katarina 2/10 - EASY your Alpha Strike out damages her abilities and you have a built in heal, just keep harrassing her down.

Akali 6/10 - pretty scary once she gets to 6 as her burst is amazing and if she stealths while you cast your alpha strike it wont hit her, in this lane you want to just farm up and gank other lanes as much as you can.

I will add more when I get more experience. :)

I hope you like AP yi please leave comments to help me further improve my guide as i am still a beginner at this. I have been playing AP yi for quite abit now so i think i have the basics and item build down. Always say GL HF and GG WP :)