•Drop W across the way out, not behind the fight, then keep the trapped target on your side of the wall with isolated Qs and an E toggle that stays on only for contact; that wall-line plus mana discipline is a huge part of real Karthus damage
•Karthus is best when the map has already started to break: full clear, buy level 6, and arrive for the skirmish that is already happening, because he is much better at collecting the second half of a fight with passive and R than at inventing a clean early gank from nothing
•Deaths can be profitable on Karthus if the enemy is already stuck finishing the play inside your wall, aura, passive, and ult, and the recent data supports that he tolerates messy high-death wins better than similar AP junglers even though 9+ deaths usually means you died before you made them pay