Been there , no fun.....Drilled a hole carefully next to the flat side of the nut on 2 sides , then PB'd into the holes and punched inside the outer loop , more PB . Then it was tighten , PB , etc. until finally a small movement , wire brushed the threads , PB and work in & out until nut was freed . You could also buy/rent a "nut-splitter" . Good luck