Depends how bad it is. You could just keep the pressure on the edge that grips best, but I guess you've already tried that.

The only thing I can think of is to try and jam something into the allen head so that it can't move and then use that to loosen it off. You got any punches that could trap on the metal? If so, do that then use a pair of plyers to twist the bolt out.

Any chance of drilling it out VERY carefully?