Maybe get vice-grips on it? Then beat it. Or, you can grind down two side to get an adjustable wrench on it. There will be more material to grab, so you can apply mroe torque. Maybe add some heat to the situation, watch the rubber brake line though.... Speaking of heat... weld something on it then get it off as a last resort. You obviously don't need to salvage the bolt, so do whatever to it. I hope this helps. I've used both techniques on other things, the first one on caliper bolts on a 91 Regal GS.
