Use a six sided socket on all "stuck" nuts and bolts!!!
If even the six sided one won't get it, then weld a large nut onto the bolt head thru the hole in the nut and then use the correct six sided socket to get that.
I will add that if you do weld a bolt to it, you want to go after getting it out as soon as you lay your welding hood down. The heat from the welding will help loosen the threads.
