The guardian you are supposed to kill for this quest is guarded by 4 priests. Each priest gives the guardian an ability, and killing a priest will prevent the guardian from absorbing that ability. The 4 abilities are:
If you are soloing this, or if there is no one else around to help you, this comment is for you. All damage abilities are very dangerous, and ideally you should try to kill as many priests as you can before the guardian awakens. If you are a class that cannot dispel magic, the Hydra priest needs to die, otherwise you have no way of outdamaging the guardian's heal.
The serpent ability can be interrupted, and should pose no threat to any class that can interrupt. If not interrupted though, it can take off more than half your health. The hawk ability can kill you quickly if you get caught in its path, since it knocks you around into other tornadoes, and each tornado can whittle your health down pretty quickly. The direhorn ability is an earthquake beneath the guardian that is easily avoided, but if you are not careful and stand in it, it deals around 40,000 damage per second.
In terms of importance and threat level, I would rate the following order in which the priests need to die: Hydra, Hawk, Direhorn, Serpent (serpent before hawk if you are a class that cannot interrupt).