My suggestion is an arcane-frost spec, playing that in a trio with a friends mage, the three of us are level 22 and mostly doing the "tag mobs, bunch up, frost nova, AE until they're dead - maybe with a (forgot the name, early level fire AE spell) after the frost nova". Once you get the mana shield and you start picking up stam+int gear from the AH it should work real well, as long as you're ready to travel to get to useful grinding spots, or if you're alliance.