Kayn vs Rammus

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.

Stay behind your minions: Powerball (Q) stops on the first enemy it collides with, minions included.

If you deal magic damage, lean on it in trades when Defensive Ball Curl (W) is on cooldown: without it, his resistances drop to those of an ordinary tank.

If you're ranged, don't let him reach melee range when Frenzying Taunt (E) is ready: its range is short, and it forces you to attack him.

Get out of the Soaring Slam (R) area fast: aftershocks repeat the slow there, and if cast during Powerball (Q), it adds a knock-up in the center.

Don't attack during Defensive Ball Curl (W): each auto returns magic damage to you, and he gains armor and magic resist.