Champions with CC to stop his healing.
Ekis the Seraph
Any champion with a form of Hard CC defeats AP Yi. (Silence, Knock Up, Knockback, Pull, Stun)
Such champions are: Soraka, Blitzcrank, Riven, Jax, Kassadin, Leona, Diana, Taric, etc.
AP Yi isn't really strong aside from his heal, stop his heal and he's done.
Hard CC, Massive Burst, Grievous Wounds are the things that come to mind.
The other option is building MR because after his Alpha Strike, AP Yi just sort of....does nothing really.
Alistar, with Trinity Force.
uh banshee's veil?
edit: just realized I'm on dominion forum ._. so odin's veil maybe lol
no, but seriously grievous wounds (Morellos/Exec's Calling) some form of interrupt CC, MR, and burst dmg
Bait his meditate then full commit after he's burned it, go.
Keep in mind that his meditate has an extremely long cooldown, and play accordingly.
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