Okay, for those of you who have come here after failing this 10 times....
You do not need to HEAL the tree, you need to keep the tree ALIVE. This subtle yet important fact means you cannot let the tree die, but you don't need to be concerned with trying to top off the HP bar either. Like the previous person said, stoneskin helps a bit. Cast holy for crowd control, and make sure to cast protect on yourself at the beginning as well. You will need to kill the mobs, as you can't heal the tree's DOT by yourself. Burn the enemies as fast as you can, while keeping stoneskin recast over and over on the tree. The NPC healers will help you recover the tree's HP after each wave.