How can I unscrew a br bolt with a stripped head while not damaging what it is screwed into?

I have a shower only system where the main parts are br. It included a br bolt to plug up the "tub plug", the threaded hole used to send the water down the to tub spout. My boyfriend put the plug in without thread tape (while I was unaware) and now with the water turned on, it leaks. We have tried and tried to take it out so we can put thread tape on but it is just super stuck. Its now at the point where the bolt head is ruined because br is soft. We've tried wrenches (regular, adjustable and pipe) with no success. Also tried hammering the wrench, he told me he tried WD-40 too. The shower system cost over 250 so I would really hate to have to replace it. Does anyone know how I can get this out? I read somewhere about heating it, but a lot of them say to use some lubricant, wax or oil, to seep in and loosen it. But this is upside down. Any help would really be appreciated! Thanks!