I've seen mobs that cast an immunity to Fire damage on them so, it seems reasonable that their are random mobs immune to all sorts of things just so that a mixed party would be the most reasonable combination in any setting.

I guess really I could set up multiple 3 Man teams. I would try melee but that just seems to be a world of pain. It's a nice idea, but I figure trying to keep multiple clones in melee range has to be a lot harder to do then just making sure they are in spell range. It's sort of the problem with MMOG's in general, I find. Range has it easier then Melee because of lag or in pvp because players hop around like madmen making it hard to get 'in melee range'.

I would like to try 2 Warriors and a Priest but I figure keeping the 2 warriors in range would be a real pita.

Maybe 1 Priest, 1 Mage, 1 Warlock. Mix it up DPS wise. Different debuffs, different attack types, etc. It's an idea.