No, don't grab the outer cap with vice grips or the like. You'll definitely damage it. It's made of a soft metal (aluminium?) Also, when you ARE removing the hatch support at the glass, it is VERY easy to cross-thread and ruin the threading. The best way to prevent this is to make sure the strut is fully extended (hatch glass all the way up) and then manually support the glass, removing the weight from the strut. You need a 12cm box-end spanner wrench to remove the nut ... as for the problem of it spinning in place ... I really don't know, as I never experienced that problem.
Anyway, hopefully some of the other info helped.