Really depends on the mob you're facing (based on our buffs more than anything). Against cold based mobs I will usually go in the primary group with the MT, as our CR buffs (and HP/mitigation buff as well) are helpful. Otherwise I go in a dps group...I imagine being with scouts/necros suits us best to max infernus seed.
Darathar is going to be pretty much immune to all your fire (fiery doom, pyro) and piercing (klicniks, swarm of bats) due to its resistance to those....dont think it has much to do with the spell level in that particular case. Haven't fought Udalan myself, but I imagine it has immunities to certain lines of spells as well.