Not very until the last two rounds. Round 4 arc burn is hell. Dismantling mines with hive wizards and Knights who have arc burn, and thrall that basically have light switch cuz their claws are arc. Much harder than the Urrox boss fight itself. All in all I'd say it's just slightly harder than Crota hard mode being that we were 32 on lvl 33 enemies. But the arc burn stage is loads harder than CE