Kog'Maw vs Yasuo

Start your trades with Caustic Spittle (Q): it reduces the target's armor and magic resist, and its ranks also give you permanent attack speed.

Activate Bio-Arcane Barrage (W) right before the teamfight: it extends your attack range, and your autos deal damage based on the target's max health.

When an enemy runs at you, cast Void Ooze (E) between you and them: the trail of ooze slows everyone who crosses it.

Save Living Artillery (R) for targets below 40% health, where it deals its max damage: each shot fired in quick succession costs more mana.

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.