Vi vs Master Yi

Hit an enemy with a spell to activate Blast Shield (passive): it charges over time, and every third hit on a target lowers its cooldown.

Hit the same target three times: Denting Blows (W) adds damage based on their max health, removes some of their armor and gives you attack speed.

Cast Relentless Force (E) on a minion in front of your opponent: the shockwave also hits enemies behind the target, with full damage.

Charge Vault Breaker (Q) out of sight before you show up: fully charged, it deals double damage, and it stops on the first champion hit to knock them back.

Cast Cease and Desist (R) on their carry: you're unstoppable during the charge, and enemies in your way are stunned. Don't stray too far from your team.

Against Highlander (R), don't run alone: regroup with your team, because he gains speed and ignores slows.

If you have CC other than a slow, land it right after Alpha Strike (Q): during the strike, he's untargetable, and Highlander (R) ignores slows.

If you have CC, interrupt Meditate (W): he channels it to heal, more at low health, and to reduce the damage he takes.

Put pressure on his jungle early on: Master Yi is strongest once he has his items, and each stolen camp delays him.