If there is nothing left to grip in the holes I would next try a left handed drill
. The vibration
, heat and left hand direction may unscrew the bolt.
Which ever direction drill
you use the hole in the bolt head will keep it centred. Start with a smaller size and drill
deep enough to be just below the head. Go up a size and drill
again but don't get to close to the bolt thread diameter. Your ideal situation will be that as you get close the head will snap off. You could drill
close then drive a screwdriver bit in and use that to snap the head off.