Be sure to move them all time. Do not dps them at all because the damage you do to them is transvered to another player and also that player will lose all their mana in seconds. They will also trow turnets into ground wich slows you. If you stay near them once they die *BOOM* (and you are dead).
They can be feared, slowed and stuned.
Can easly kited by ranged classes plus Crippling Posion or Hamstring. As long as they move, they do not cast their heavy DPS spell and thus kiting them along the big circle is the easiest way to kill them.
Best are Hunters plus one Rogue. The Rogue using Shiv and Crippling Posion (do not pull aggro off the Hunters), and the Hunters Aspect of the Cheeta. If you killed all other mobs, let the Hunters Feign Death, and the mobs run to the next healer where tanks can taunt and stun them.
Energy Sap: Dealing 2000 Arcane damage per second to nearby friends. Draining 0 Mana per second from nearby friends. Draining 0 Energy per second from nearby friends. 6 seconds remaining