Fizz vs Master Yi

Last-hit minions with Seastone Trident (W) early on: if the empowered auto kills, you get mana back and its cooldown drops.

Save Playful (E) to dodge a key spell: you're untargetable for 0.75 seconds, then land with damage and a slow.

Cast Chum the Waters (R) from range: the farther the fish travels before it attaches, the stronger the shark's slow and damage.

After Chum the Waters (R), chain Urchin Strike (Q) and Seastone Trident (W) while the target is slowed: the shark then bursts up under them and knocks them into the air.

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.