This only happens when I try to get the troll skin to make the ulu-mulu.
Are you alone or with Diego ?
If Saturas gave you the 'Search for the Focus Stones' quest, when you go to Focus Canyon (the place where the Troll is), Diego will be there to help you fight the goblins, the Troll and the Harpies that are guarding the Focus Stone.
Diego and the Troll are kind of 'immortal', so you just have to let them fight each other, approach the troll from behind, get close enough to see his name on the screen, and cast the spell.
I'm under the impression that in order for the spell to work, the troll can't be aware of you, i.e. attacking you.