Watched someone play AP Master Yi just now. He decimated the enemy team. However, I can not vouch for the abilities of the enemy team and how much of a fluke this was. However, what caught my eye was:
Alpha Strike: He use this and hit multiple enemies. He'd take AP characters down to half health and tankier (Olaf) champs down to 65% health. So, he'd alpha strike, then run away and wait for it to cool down, strike again, etc.
Meditate: I watched him fight lux and Teemo under a turret. He killed Lux and started hitting teemo. Then, he started to meditate. He was getting hit by teemo AND their turret, and he was healing. He healed faster while getting hit by those two things, than I do sitting in our spawn. He was full health and still meditating with Teemo and the turret still hitting him. He alpha'd again and hit teemo the last couple hits to kill him. Then ran out from under the turret.
I've never been a fan of off damage builds for characters. But the way he played him, it really worked out. He didn't play him as an AD carry. He played him as a squishy AP character. He'd alpha, then move away. Alpha, then move. Meditate. Repeat. His fast moving ult seemed to last a life time.
He ended the game 19/5/13.
Now, they had a morg and a lux. So, they had plenty of CC to stun him and shut him down. Not sure if they just weren't, or if he would just meditate through their stuns and alpha out of it. So, again, this could have been a fluke because of the enemy team. However, the enemy team didn't have anything to do with his turret tanking ability during meditate. I couldn't believe it. An end game teemo and a turret hitting him, and he's gaining health faster than I do sitting in our spawn.
Anyway, his build was:
Deathfire Grasp
Zhonya's Hourglass (this might have helped during stuns too)
Void Staff
Rabadon's Deathcap
Lich Bane
Sorcerer's Shoes
Flash and Ignite