Zhonya's, Edge of Night, and Banshee's block/cancel his R — buy one of these items if Karthus focuses you.

His Q deals double damage on isolated targets: stay in your minions or near allies to split the damage.

Shields, Soraka R, and heals can counter his R if well-timed — save an emergency heal for Requiem.

Karthus is immobile and squishy — dive him quickly to eliminate him before he deals continuous damage.