Next to anything is possible. 5x dps, such as 5x ele shaman or a warlock/spriest combo. For leveling, this will cut it for sure.
However, a setup without a tank will mean that you either have to crush a target so fast that your toons do not die, or kite them.

I prefer to do dungeons with a normal group setup: Tankadin (aoetanking capability goes well with aoe-casters), Mage (sheep!!, biscuits, portals, AI) 2xWarlock (HS, SS, DD, AoE) and holy/disc priest.