Cassiopeia vs Yasuo

Land Noxious Blast (Q), then chain Twin Fang (E) while the poison lasts: the poisoned target takes extra damage, and you heal.

Hit a champion with Noxious Blast (Q) to kite: the spell gives you move speed, and Serpentine Grace (passive) makes every speed bonus more effective.

Place Miasma (W) on those trying to jump on you: inside the clouds, enemies are slowed and can't use movement abilities.

Cast Petrifying Gaze (R) toward those chasing you, after a Flash if needed: those facing you are stunned, the others only slowed.

Hit him with a cheap attack to trigger the Way of the Wanderer (passive) shield, then start the real trade: otherwise, it absorbs your best damage.

If you have projectiles, save them for after Wind Wall (W): it blocks them all for 4 seconds, on a 25 to 17 second cooldown. Other spells go through it.

Don't stand right behind your minions: each one gives him a target for Sweeping Blade (E), and he gets closer to you with every dash.

Get out of the line of his whirlwind, the third Steel Tempest (Q): without a knock-up, he can't cast Last Breath (R).

Judge his threat by his crit items more than his level: Way of the Wanderer (passive) raises his critical strike chance, so each crit item does more for him.