If you can, get some heat on it.....presumably its rounded ccw see if you can tighten it slightly this can sometimes break the crud thats holding it and it 'might" undo.
If that doesn't work id get it to a garage and get something tacked on to undo it..again welding gets it hot and usually a bit of heat works a treat.
If you go down the drill and extractor route be very carefull a broken off tool steel extractor in there adds a world of pain to a job.
Good luck.