Mordekaiser vs Nasus

Aim Obliterate (Q) at a lone enemy: if only one enemy is hit, the damage increases.

Hit champions or monsters three times, then stay close: Darkness Rise (passive) then surrounds you with a damage aura and gives you move speed.

Pull your targets into the Obliterate (Q) zone with Death's Grasp (E): each spell that hits also counts toward Darkness Rise (passive).

Cast Indestructible (W) when you're taking damage: it turns stored damage into a shield, and a recast converts part of it into health.

Cast Realm of Death (R) on their carry, or their jungler near an objective: you isolate them with you, steal part of their stats, and keep them if you kill them.

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.