Kha'Zix vs Xin Zhao

Jump on an isolated target with Leap (E), then Taste Their Fear (Q): with no ally nearby, Unseen Threat (passive) marks them and the spell hits much harder.

In teamfights, cast Void Assault (R) to vanish and reposition: the invisibility triggers Unseen Threat (passive), and your next auto damages and slows.

Detonate Void Spike (W) close to you in the jungle or in teamfights: you heal if you're inside the explosion area.

Pick your evolutions to fit the game: evolved Leap (E) resets on each takedown, evolved Taste Their Fear (Q) gains range and a lower cooldown on isolated targets.

Get out of the line of the Wind Becomes Lightning (W) thrust: it slows you and marks you as Challenged, which extends the range of Audacious Charge (E) on you.

If you have CC, use it during Three Talon Strike (Q): without his three autos, there's no knock-up, and his other cooldowns don't go down.

After Audacious Charge (E), keep your distance: it's his only dash to get back on you, and without it he has to walk up to you.

After Crescent Guard (R), don't hit him from range: for a moment, he ignores damage from outside the swept zone. Back off, or hit him from inside it.

If you survive his first minutes, play for time: Xin Zhao is strongest early on, through his ganks and skirmishes.