I JUST finished boosting 2 teams to 60 with a 74 Paladin and everything went fine, so I'm sure one of the above posters nailed it with the idea that you are using non-player damage to kill the mobs. This would include damage from a totem or a pet. You can still use these damage types, but they cannot be the sole damage dealer.

With a Shaman, you could put on a shield spike or cast Thunderstorm once.
With a Warlock, a Seed of Corruption or Shadow Fury will do the trick.
With a Hunter, Arrow Shower will work, but you may be better off not AEing with a hunter, and just running through and single targetting or Multi-Shotting groups of 4.