Im not sure which bolt you mean, size or location. But in the past i have had some mild luck with screw extractor drill bits. I dont know if you have the space for that setup. Another thing if its really stubborn you can grind two sides of the head and form it into a square or rectangle and throw a crescent wrench on it. But again that requires space and an exposed bolt head. Be very careful if you go the grinder route. Sparks and such haha. Im sure someone else will chime in with more assistance.