Fiddlesticks vs Master Yi

Channel Crowstorm (R) out of their vision, from a brush or behind a wall: if you're unseen when your spells hit, Terrify (Q) fears them too.

Aim the center of Reap (E) at the key target: in the center, they're silenced; at the edges, only slowed.

Channel Bountiful Harvest (W) to the end on camps and minion groups: you heal for part of the damage, and the remaining cooldown drops by 60%.

Place the effigies from A Harmless Scarecrow (passive) in key brushes: they replace your trinket, and Terrify (Q) can fear when you're mimicking one.

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.