Karthus vs Rammus

Aim Lay Waste (Q) at a champion away from their minions: if the blast hits only one enemy, it deals much more damage.

Place Wall of Pain (W) in their path before your combo: anyone who crosses it is slowed and loses magic resist.

After you die, keep casting your spells: Death Defied (passive) keeps you in spirit form for a while, and you can still cast Lay Waste (Q) and Requiem (R).

Save Requiem (R) to finish off wounded enemies, often right after a teamfight: it hits every enemy champion wherever they are, after a 3-second channel.

Stay behind your minions: Powerball (Q) stops on the first enemy it collides with, minions included.

If you deal magic damage, lean on it in trades when Defensive Ball Curl (W) is on cooldown: without it, his resistances drop to those of an ordinary tank.

If you're ranged, don't let him reach melee range when Frenzying Taunt (E) is ready: its range is short, and it forces you to attack him.

Get out of the Soaring Slam (R) area fast: aftershocks repeat the slow there, and if cast during Powerball (Q), it adds a knock-up in the center.

Don't attack during Defensive Ball Curl (W): each auto returns magic damage to you, and he gains armor and magic resist.