Zac vs Master Yi

After Stretching Strikes (Q) on one enemy, hit a second enemy with your auto: you fling them into each other, and if they collide, they and those nearby take damage.

Pick up the chunks of Goo you shed when your spells hit: Cell Division (passive) heals you for each one, and Unstable Matter (W) comes back 1 second sooner.

Charge Elastic Slingshot (E) from the fog of war: the enemy has less time to react, and the longer the charge, the longer the knock-up on landing.

Cast Let's Bounce! (R) into the heart of their team: the first bounce knocks back each enemy hit, the following ones slow them, and you can cast Unstable Matter (W) at the same time.

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.