Nasus vs Mordekaiser

Last-hit minions with Siphoning Strike (Q): each kill with the spell permanently increases its damage, by more on large minions and champions.

In teamfights, cast Wither (W) on their carry: the slow grows stronger over the spell's duration and also reduces their attack speed.

Cast Fury of the Sands (R) as you enter a teamfight: for 15 seconds, you gain health and resistances, damage nearby enemies, and Siphoning Strike (Q) comes back faster.

Early on, farm rather than force trades: Siphoning Strike (Q) stacks are permanent, and every minion you last-hit makes you stronger later.

Stay near your minions against Obliterate (Q): its damage increases when it hits only one enemy.

Dodge Death's Grasp (E): it pulls you toward him, into his other spells, and it has a 16 to 8 second cooldown.

If you have range, kite him: Mordekaiser has no dash, and only Death's Grasp (E), which pulls you in, closes the distance.

In Realm of Death (R), save your mobility spells to escape him rather than for the duel: he steals part of your stats, and keeps them if he kills you.