Jarvan IV vs Master Yi

Plant Demacian Standard (E), then cast Dragon Strike (Q) toward it: you fly to the flag and knock enemies in your path into the air.

Land an auto as soon as you reach your target: Martial Cadence (passive) adds damage based on their current health, then only triggers on them again after a few seconds.

Cast Cataclysm (R) on a carry who is away from their team: the arena is impassable, and you can recast to collapse it if it gets in your allies' way.

In teamfights, plant Demacian Standard (E) near the allies who are attacking: it gives them attack speed while they stay close to it.

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.