If you can get a wrench (or vice grips or whatever) on the nut, just twist the bolt off and knock the remains out of the hole. That's how I got my severely rusted fender bolts loose. I did get a few with a clutch head socket adapter and a socket on the nut, but most I just twisted off. You could probably use the same method on the head of the bolt, but if you can't hold on to the nut to back it up, the whole thing may start turning in the hole. Even if the bolt is into a threaded hole (the part is the nut), twist the head off to get the part disassembled, then drill out the remains of the bolt and either chase the threads or install a helicoil.
