Xerath vs Nasus

Charge Arcanopulse (Q) to poke from long range: its range grows with the charge, but you slow down by up to 50% in the meantime.

Slow your target with Eye of Destruction (W) before Arcanopulse (Q): the center of the zone slows the most, and a slowed target dodges poorly.

Land Shocking Orb (E), then center Eye of Destruction (W) on the target: stunned, they can't dodge it, and the center slows the most.

Cast Rite of the Arcane (R) to finish a fleeing target or support a distant fight: your shots reach 5000 range, with bonus damage per champion hit.

Early on, take a trade whenever he steps up to last-hit a minion: each kill with Siphoning Strike (Q) increases its damage for the whole game.

Freeze the wave near your tower: to farm with Siphoning Strike (Q), he then has to step up and expose himself to ganks.

Once he hits level 6, don't engage during Fury of the Sands (R): for 15 seconds, he gains health and resistances, and his area damage scales with your max health.

In teamfights, if you're ranged, hit him while backing off: he has no dash, only Wither (W), a slow that grows stronger over time.