Karna or shishou

Karna = farmer
Shishou = divine boss killer

I'd say shishou, but I might be biased since I've had her since her first appearance. There are plenty of options for farming any given stage, but she brings a high powered single target stun to the table, the potential for massive crit damage, and up to 100% more attack vs divine enemies on top of that. She also has a targeted Quick card buff which is pretty rare. She's one of the more reliable boss killers in the game at the moment.

Karna can do some serious crit damage himnself and can partially charge himself with his skill, but for boss battles shishou would be my pick.

They both do the same general thing, kill divine enemies, except Scathach is ST and Karna is AoE, therefore Scathach is better in big boss fights and Karna is better at wave clearing.