Fiberglass and metal do not mix. Adding fiberglass to metal is hack repair 101, and a sure sign of substandard work. Always. There's a right way to do it, and that's not it.
Propane torch and a wet rag should work just fine. Heat it with the torch, cool it with the wet rag, should shrink it up pretty well. You don't need the metal red hot either, just hot. And you shouldn't need to heat the whole door skin, just the area of the dent. If done correctly you shouldn't need anything behind it. Easier said than done of course, there's definitely skill to shrinking the metal the right amount.