stripped or rounded off the outside of the nut? I'm guessing you rounded off the nut, so I would suggest using a dremel to grind two opposite sides of the nut flat so you can get a wrench on there. Make sure you have two very deep solid flat spots before you try loosening, or you'll just round the edges off again. Then spray the hell out of the nut with WD-40 or something similar, let it sit for a few hours, spray it again, then hammer hard on the bolt and try to get it to loosen. Could try having someone hammer on the bolt while you are torquing on it too.
Next time use the proper box-end wrench or the right sized socket...