Sett vs Mordekaiser

Start your trades with Knuckle Down (Q): you rush toward your target, and your next two autos deal bonus damage based on their max health.

Cast Haymaker (W) right after taking damage: the stored Grit gives you a shield and extra damage, dealt as true damage in the center of the punch.

Don't back too early at low health: Pit Grit (passive) increases your health regen based on your missing health.

Stand between two enemies before Facebreaker (E): grabbed from both sides, they're stunned, otherwise only slowed.

Grab their tank with The Show Stopper (R) and slam them onto their carries: the damage around the impact depends on the bonus health of the target you grab.

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.