Welcome! I use WD-40...or dip the figure or torso part in boiling water until I can unscrew it. I prefer to use WD40, because the boiling may slightly discolorate the plastic.
EDIT: Oops. Read a second time and you specified "stripped screw". I actually drill through the head of the screw, eliminating it. Once the head is gone, It's easy to take apart the two torso's and you use a good plier tool to manually unscrew what's left inside the plastic. Works every time.