" I've stripped two valve stem wrenches trying to loosen the hot water
valve in this shower.

Make sure you have the correct size stem wrench, and that it has a deep
socket construction. A slightly oversized wrench will appear to strip, when
it is actually a mite too large. Once you are assured the wrench fits
right, tap the cross bar with a hammer to jar the stuck threads loose. As
the other poster says, beware if stem is supported only by piping, as you
may cause a pipe joint leak by overtorquing the stem nut.