I'm a very bad player(Platinum 2 on EUW)
My mecanic are the worst(like my english lol(and my writing skills)) so I thought about a build which deal dmg mainly with karthu's e.
Fight with your wall , mr reduction is all.
It's too hard to land q on ennemy
Rather funny to play ,approximalty same amout of dmg as a normal karthus(but not in a burst way so can be useless agaisnt high regen comp)
Don't expect your R to deal half hp bar
Farm is all,karthus become more a statschecker.

Conqueror is fine is you land all of your q because your e don't stack conqueror enough
You don't need CD on your spells becuase karthus e is an aura
So Archangel isn't a good choice
Runes and catalyst passiv is enough mana