Kayn vs Warwick

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.

Keep your health above 50%: below that, Blood Hunt (W) gives him speed toward you and attack speed, three times more below 25%.

When he drops below 50% health, finish him fast, with CC if you have any: Eternal Hunger (passive) then heals him with each auto, three times more below 25%.

If you have a dash, don't count on it to escape his held bite: Jaws of the Beast (Q) makes him follow all your movement, then leap behind you.

In teamfights, stay behind your tankiest allies: Infinite Duress (R) suppresses the first champion hit, and its range grows with his move speed.

If you have Grievous Wounds, apply it to him: his autos heal him at low health, and Infinite Duress (R) heals him for all the damage it deals.