Graves vs Karthus

Position with no minion between you and your target: each New Destiny (passive) auto fires 4 bullets that can't pass through units.

Cast Smoke Screen (W) on their carries: inside the smoke, they're slowed and can't see anything outside it for 4 seconds.

Fire Collateral Damage (R) with your back to the danger: the shot knocks you backward, enough to reposition while dealing damage.

Cast Quickdraw (E) often during your clear: it reloads a shell, gives armor and magic resist, and each bullet that hits lowers its cooldown.

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.