U max e for the reason that it is very hard to land ur q. And it is actually fun to watch your opponent die slowly while u die faster.

Try not to fight due to karthus is very weak at 1v1.

Tell your jungler to gank more.

Don't push the lane, unless u r fed.

Get assist or kills from ulting in order to get fed.

***DONT GET KILLED***

In teamfight, try to flash in or get into the middle, activate Seraph's Embrace or zhonyas hourglass, exhaust their most fed person, stay alive.

Once u die keep q and w, activate ultimateat the last posible second.

Karthus is very devastating when enemy is doing baron/ dragon, due to the fact that they are grouped up.

U can do absurd dmg.