Kayn vs Trundle

Take Rhaast against a tanky team, Shadow Assassin against squishy targets: The Darkin Scythe (passive) gives one healing, the other bonus damage at the start of fights.

Gank through walls with Shadow Step (E): you move through terrain faster, and you heal when you first enter it.

To dodge a burst, cast Umbral Trespass (R) on a champion you just hit: inside, you're untargetable while your spells come back up.

As Rhaast, hit as many champions as you can with your spells: The Darkin Scythe (passive) heals you for part of the ability damage they take.

Don't fight in Frozen Domain (W): he gains move speed, attack speed and increased healing inside. Back off to draw him out of the zone.

If you deal physical damage, back off after his Chomp (Q): it steals some of your attack damage for a few seconds, and he gains it.

Get away from Pillar of Ice (E) as fast as you can: it slows nearby enemies and blocks the path while it stands.

Back off when he casts Subjugate (R) on you: he steals part of your armor and magic resist, and gains what you lose.