Kayn vs Bel'Veth

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.

Back off when she takes her true form with Endless Banquet (R): she gains health, range and attack speed, and with enough stacks, the form lasts until she dies.

Dodge Above and Below (W): if it hits you, you take a knock-up and her Void Surge (Q) in your direction is refreshed instantly.

If you're the lowest-health enemy near her, get out of Royal Maelstrom (E) range: every slash targets you, and she gains lifesteal and damage reduction.

Deny her large monsters and kills: Death in Lavender (passive) gives her permanent attack speed for each one.