Sejuani vs Master Yi

Land both Winter's Wrath (W) swings: the second deals most of the damage and slows, and each one applies a Frost stack.

Cast Permafrost (E) as soon as your target has 4 Frost stacks: Winter's Wrath (W) and your melee allies' autos apply them.

Hit a target you just stunned: Fury of the North (passive) shatters them for heavy magic damage.

Engage with Arctic Assault (Q): enemies you pass through take a knock-up, and the charge stops on the first champion hit.

Cast Glacial Prison (R) at their carry from far enough away: past a quarter of its range, the stun lasts longer and a storm slows nearby enemies.

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.