Diana vs Master Yi

Hit with Crescent Strike (Q) before Lunar Rush (E): on a target marked with Moonlight, the dash has no cooldown and you can recast it right away.

Weave autos between your spells: after each ability, Moonsilver Blade (passive) boosts your attack speed for 5 seconds, and your third hit cleaves nearby enemies.

Cast Pale Cascade (W) before you engage: you gain a shield, then a second one when the last sphere explodes.

Cast Moonfall (R) in the middle of their team: it pulls in and slows nearby enemies, and hits harder for each champion pulled after the first.

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.