Rammus vs Master Yi

Engage with Powerball (Q): your speed builds while you roll, and the impact knocks back and slows enemies around your target.

Cast Defensive Ball Curl (W) when you're being hit by autos: you gain armor and magic resist, and each attack returns magic damage to its attacker.

Cast Frenzying Taunt (E) during Defensive Ball Curl (W): forced to attack you, your target takes damage with every auto.

Cast Soaring Slam (R) during Powerball (Q): enemies in the center of the area also take a knock-up, and your speed extends the leap's range.

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.