Kassadin vs Yasuo

Against a mage, take your trades with Null Sphere (Q): it gives you a magic damage shield for 1.5 seconds and interrupts channels.

Restore mana with Nether Blade (W): your next auto restores part of your missing mana, more on a champion.

Farm safely early on: your power comes from ranks in Riftwalk (R), whose cooldown drops from 5 to 2 seconds.

Chain Riftwalk (R) in teamfights while watching your mana: each quick recast adds damage, but doubles its mana cost.

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.