Miss Fortune vs Karthus

Fire Double Up (Q) at a dying minion in front of a champion: if the shot kills the minion, the bounce onto the champion always crits.

Switch targets often: Love Tap (passive) adds damage on each new target, and lowers the cooldown of Strut (W).

Activate Strut (W) to get in position before Bullet Time (R): you instantly gain its max move speed bonus, plus attack speed.

Cast Bullet Time (R) on enemies who are CC'd or bunched up in a chokepoint: it's a channeled cone, and each wave can crit.

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.