Shaco vs Karthus

Gank with Deceive (Q) and come in behind your target: Backstab (passive) empowers hits from behind, and your first invisible auto becomes a critical strike.

Save Two-Shiv Poison (E) to finish: the shiv deals much more damage to a target below 30% health.

Cast Hallucinate (R) to bait their spells: you briefly disappear and reappear with a clone, which explodes on death into three boxes that fear.

Place Jack In The Box (W) on their escape routes or at a camp entrance: invisible after a delay, it fears champions who come near, then shoots them.

Stay near your minions or your allies: Lay Waste (Q) deals much more damage when the blast hits only one enemy.

If you can engage, jump on him: Karthus has no dash or leap to escape.

Once he hits level 6, back instead of staying at low health: Requiem (R) hits every enemy champion wherever they are, 3 seconds after he starts channeling.

If you have a shield, a heal or invulnerability, save it for Requiem (R): it hits every enemy champion 3 seconds after he starts channeling.

Once Karthus is dead, move away: Death Defied (passive) keeps him in spirit form for a while, and he can still cast his spells.