Darius vs Master Yi

Aim the outer edge of Decimate (Q): only the blade heals you and applies Hemorrhage (passive). The handle, in the center, deals less damage.

Go all-in when your target has five Hemorrhage (passive) stacks: you gain a massive AD bonus, and Noxian Guillotine (R) hits harder with each stack.

Save Apprehend (E) to pull back an enemy who retreats, not to start a trade: it's your only pull, on a 26 to 16 second cooldown.

Cast Noxian Guillotine (R) on the lowest-health target: its true damage ignores armor, and if it kills, you can recast it right away.

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.