I've stripped two valve stem wrenches trying to loosen the hot water
valve in this shower. I've tried wd40 and heating it up with a torch
to get it to loosen up but no luck so far. (I did get the cold stem
to give after much grunting and some cursing)

The shower is one piece fiberglass and it backs up to another one
piece fiberglass shower so I very much don't want to break anything
loose inside the wall.

Are there any special techniques/tools to help with this situation?
Some sort of special wrench or magic song to sing or something? I'm
afraid to try another standard valve stem wrench because I'm afraid
I'll start to round off the nut inside the wall and then never be able
to get a grip on it.

Any shared experience would be greatly appreciated!

John