Depending on how much access you have to this bolt...

You might try the hammer and chisel route. Get a good chisel with a
edge smaller than the diameter of the bolt. Use a dremel to cut a slot
in the top of the bolt. Then put the chisel into one side of the bolt
and hammer the chisel (in the direction of removal). Worked for me when
I had a stuck bolt on my brakes (yes, the bolt will be trash
afterwards.)