Match the screw size to a left hand drill bit like this http://www.amazon.com/Forney-20481-Industrial-Machine-16-Inch/dp/B003XF3QYQ/ref=sr_1_11?ie=UTF8&qid=1424886742&sr=8-11&keywords=left+hand+drill+bit
Use a drill that will reverse, slowly drill the head of the screw. With luck it will "catch" and unscrew the screw. If not, keep drilling just enough to remove the head so you can remove the part and expose the rest of the screw. Then use small pliers and unscrew it. Or leave it if its not critical.