Gragas vs Master Yi

Let Barrel Roll (Q) charge before detonating it: damage and slow grow the longer it sits on the ground, and if you don't recast it, it explodes on its own.

Drink Drunken Rage (W) before engaging: drinking takes 1 second, then you take less damage and your next auto hits every nearby enemy.

Engage with Body Slam (E) through a path without minions: you stop on the first enemy you meet, and hitting reduces its cooldown.

After Body Slam (E), cast Explosive Cask (R) behind your target: the blast knocks enemies away from the impact, sending them toward 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.