Penatrating oil/heat (propane torch) and some vise grips might get it.
Or you might see if you can get a stud remover/installer socket from Harbour Frieght and use that on what's left of the bolt head.
Hammer and a chisle, and try knocking it in a counter clock wise direction.
Drill and an EZ-out.
That's all that comes to mind right now.