Jinx vs Karthus

Poke with Fishbones, the rocket launcher of Switcheroo! (Q), for its range and splash damage, then swap back to the minigun for long trades.

Fire Zap! (W) to kite the bruiser running at you or catch an enemy who's fleeing: it slows and reveals the first enemy hit.

Throw Flame Chompers! (E) in the path of their engage or their escape: they root the champion who steps on them and last 5 seconds.

In teamfights, focus the lowest-health target first: each takedown triggers Get Excited! (passive), a big boost to attack speed and move speed.

Fire Super Mega Death Rocket! (R) to finish a fleeing enemy rather than to start a fight: its damage grows with their missing health and over its first second of flight.

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.