His gameplay pattern was,
go into battle, pop R if they're low, then Q > Did they die? if yes Q again, if not press W and become unkillable until your Q goes off cd, rinse and repeat.
Playing against ap yi was pure torture, as half of the teamfight he spent being untargetable while dealing RIDICULOUS aoe damage (and it was back when Q aoe wasn't limited to a small area like it is now so kills 1100 range away, wasn't unheard of because of RNG jumps) and the other half he spent meditating gaining so much resistance you couldn't burst him, and healing for 400% ap so there was no way for you to dps him down.
Some teamcomps couldn't actually deal with him at all, as they needed a champion who'd survive his burst that had reliable hard cc, and even then he could just play cleanup crew and rundown your team after a teamfight to score a penta.
Add to that his laning which was stupid for a champion scaling so hard, as he Q'd a wave ONCE to kill all of the minions in it, then backed away.