Hi guys, i wanna tell u about AP Yi.
My english not perfect,and this my 1 Guide, but i try write all what i can, thx ^^
Some People Think what he useless in teamfight,but it not true.
For sure - Ap Yi - The Best AOE dmg in teamfight. U ask why the best?
Bcs if he kill some 1 - he can Q again and again.
I have ~28 pentakills by AP Yi.

MID)] 1. Never pull blue, let ad cary or Top do that, bcs u can easy kill creeps with Q.
2) When u come on mid - u need fast stuck passive, when use Q and 1 autoattack on champion, it will deal 150~+200 dmg. It realy nice dmg from 1 Q.
3) On lv 2 up your E, stuck passive and do it again with active E.
4) If enemy strong and deal alot of dmg 2, ALL TIME USE Q ON 3 CREEPS.
5) For sure if your enemy dont have any control, u can easy dive with passive Q and Ignite him.

TOP) Do all what u do on mid, but know, what on top alot of champions with control can kill u.

BOT) 1. Never farm, bcs your ad cary need more farm when u.
2) 1 item LuckyKey, it give u alot of gold + dmg
3) Use Q only on 3 creeps or on enemy champion.

TEAMFIGHT.) In teamfight - try 2 kill ANYONE, and u can kill all enemys , it realy great AOE. Perfect - if your teamfigth starting without creeps, it mean what with Q u dmg 4 enemys , and if u kill some 1, with 1-2 Q u can easy carry the kills :D

Your Realy Harras starting with Rabodon and Lichbane.
If u sure what u can good play this game, buy Mejai.
With 700~ ap u can 1 shot any ad cary.
It mean 1000 with Q + 300 passive + 700 lichbane.
2-3 Month ago i see the bug with lichbane, if u have passive and use q u deal ~1000 with q and 700+700ap + 300 passive = ~2500~2700 dmg from 1 Q. But it fixed :[

Never Go in TeamFight 1. Realy try kill some 1 or steal some 1 xD
Use Q on ad cary or mid champions, bcs u have more chane 2 kill they.

So, gl and hf.