Nasus vs Master Yi

Last-hit minions with Siphoning Strike (Q): each kill with the spell permanently increases its damage, by more on large minions and champions.

In teamfights, cast Wither (W) on their carry: the slow grows stronger over the spell's duration and also reduces their attack speed.

Cast Fury of the Sands (R) as you enter a teamfight: for 15 seconds, you gain health and resistances, damage nearby enemies, and Siphoning Strike (Q) comes back faster.

Early on, farm rather than force trades: Siphoning Strike (Q) stacks are permanent, and every minion you last-hit makes you stronger later.

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.