Dr. Mundo vs Master Yi

Poke with Infected Bonesaw (Q): it slows the first enemy hit, deals damage based on their current health and restores your health.

Pick up the Goes Where He Pleases (passive) canister after resisting a CC: it restores health and lowers the passive's cooldown.

Recast Heart Zapper (W) next to a champion: the burst heals you for a bigger share of the stored damage if it hits a champion.

Cast Maximum Dosage (R) once you've taken damage, not before the fight: the instant heal depends on your missing health, then you regenerate part of your max health.

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.