well, sometimes magic can't help. like a human with a terminal illness, magic cannot cure it. it can help ease it, but cure it probably not.
do you know what's wrong with the tree? there could be something to help it already. you could try making some kind of healing brew, mix in some 20-20-20 [plant food] and pour it around the base of the tree. keep up with the healing energy though, try meditating under it to see if you can connect with earth better and give it more energy. just remember, everything dies eventually, sad as it might be, at some point, you've got to let nature take its course.