0
A couple of other things to try for posterity.. you can use a Dremel to cut a line in the head of the bolt, then use a big flathead screwdriver to undo it. Or you can use a Dremel to grind the sides of the bolt head down and make it into something close to a hex-head bolt, and then remove it with a wrench.
The latter works well if you round something that would use, say, a 14mm wrench; you grind it down into something that you can undo with a, say, 13mm wrench.