I was going to suggest a LH drill, better though would be a LH endmill (Clamp job on drilling machine, after centering up, then HAND rotate cutter whilst applying firm pressure with the quill, screw should back out.
actually a screwdriver shaped blade (with sharp edges) might work.
Another method is to get the screw hot to soften the grip in the substrate using a soldering iron or just repeated applications of say gas ring heated old screwdriver, then use sharp blade as above.
Ian P