Trundle vs Mordekaiser

Start your trades with Chomp (Q): the empowered auto slows your target, then steals some of their attack damage, which matters most against a physical damage dealer.

Take your trades near the wave: King's Tribute (passive) heals you for part of the max health of each enemy that dies near you, minions included.

Place Frozen Domain (W) where the fight will happen and draw the enemy into it: inside, you gain move speed, attack speed and increased healing.

Place Pillar of Ice (E) behind the fleeing enemy: it blocks their path and slows nearby enemies.

Cast Subjugate (R) on the enemy tank: you steal part of their armor and magic resist, and your team can focus them down.

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.