My best booster has been my level 90 Shaman. My lowbies stay on follow and I just cast chain lightning on everything and the packs of mobs die before the even reach me. Mind you, I don't run in and try to aggro everything. I just go from pack to pack all the way to the end boss of the instance. I have found that rounding the mobs up and killing them all takes just about as much time as killing the packs one at a time. And in some cases, it's just not feasible to round up mobs. Like in Halls of Lightning. The mobs toward the end will wreck you because they constantly interrupt. A Druid would probably be best in that instance if your strategy is to round up the mobs.