Yasuo vs Viktor Mid Matchup

Why Viktor is favored

Viktor counters Yasuo because his Siphon Power empowered basic attack completely bypasses Wind Wall, while defensively placed Gravity Fields deny Yasuo's dashes and allow Viktor to kite and whittle him down from range.

How to play as Yasuo

Run Doran's Shield with Second Wind to absorb Viktor's constant ranged poke, and look to all-in him only after baiting him into spending his mana pool on the wave.

When engaging on Viktor with Sweeping Blade, place Wind Wall at point-blank melee range to force him to fight around the barrier and block his return abilities while you trade.

Wind Wall intercepts Hextech Ray upon contact, destroying the cutting beam projectile before it can reach its target path.

How to play as Viktor

Pop Yasuo's Way of the Wanderer shield with a simple basic attack or laser tip before unleashing your full combo to ensure your heavy trade damage hits his actual health bar.

If Yasuo places Wind Wall to block your poke, cast Siphon Power on a nearby minion to gain your shield and movespeed, then hit Yasuo with the empowered basic attack which ignores the wall entirely.

When Yasuo dashes at you, move away from your minions and drop Gravity Field directly at your own feet: this forces him to either disengage immediately or get stunned without minion escape routes.

The initial missile of Siphon Power is blocked by Wind Wall, but the subsequent empowered basic attack is a non-projectile that deals full damage straight through Wind Wall.

Gravity Field is a grounded area-of-effect zone rather than a projectile, so Yasuo's Wind Wall cannot block or remove it.

Sources

These notes come from r/viktormains and r/YasuoMains, checked against the champions' abilities.