Hecarim vs Master Yi

Cast Devastating Charge (E) far from your target: the longer the run, the farther the knockback and the harder the hit, and Warpath (passive) turns your bonus move speed into AD.

Cast Onslaught of Shadows (R) from range: the fear from the shockwave, at the end of the dash, lasts longer after a long run.

In teamfights, recast Rampage (Q) as soon as it's back: each cast that hits adds a stack, which lowers its cooldown and raises its damage.

Activate Spirit of Dread (W) in the middle of the enemies: you gain armor and magic resist, and heal for part of the damage you and your team deal to them.

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.