had this problem once before...the solution [for me] was to do:
put the motor into my hand, with the backplate, deep into my palm.
Pull my thumb around one side of the flywheel and the middle finger and point finger around the otherside of the flywheel and pull back as hard as you can, then while your squeezing the flywheel back, try again to loosen the screw.
If you stripped the head of the screw, use a dremel to cut a straight notch in the head of the screw [make it deep] and use a straight screwdriver.
What this does, is 'wedges' the flywheel into the collar, since it 'crushes' together as an assembly on a tapered seat. This should [if your hands are built like mine] squeeze around the crank tight enough to keep it form slipping.
Hope this helps!