Veigar vs Yasuo

Last-hit minions with Baleful Strike (Q): each kill with this spell gives you a Phenomenal Evil stack, which is power for the whole game, and large minions give more.

Place Event Horizon (E) so your target is on the edge or has to cross it: only the cage's perimeter stuns, and it only lasts 3 seconds.

Stun first with Event Horizon (E), then cast Dark Matter (W): a stunned target can't dodge it.

Cast Primordial Burst (R) last, after your other spells: its damage rises with the target's missing health and peaks below 33%.

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.