I have 4 mages and a priest, and I make good use of Arcane Explosion and Blast Wave. If you can do blizzard/flamestrike, then more power to you. With my single mage, I specced him PoM Pyro, and did a PoM Flamestrike followed by blizzard to some great effect. If you can keep the mobs either frost nova'd in place, or have a pally tank keep em busy, you can obliterate most anything in short order.

But you can't AoE Everything, as it's not the most mana efficient way to go. I tend to wipe if I get adds, or if the mobs are too high and start resting a lot. Because of this, I also make good use of Wands, which when you have 5 going at once, do pretty significant damage without using any mana. I use it for normal grinding single mobs while i wait for mana regen.