Take out 40 Below and use his Wrecking Ball technique on Thorn the entire time. Do not worry about any of the enemy trolls that Kalus throws around you. Just keep on Thorn with the wrecking ball and go in circles around him so that he and
the other monsters cannot get any good shots in at you. When you have Thorn dead, Kalus will appear and try to revive him. As before, do not worry about all the other monsters. Just transform into Chilla and use his Ice Spears to take her out.
You may need to do this two or more times if she revives Thorn.