I have a tool for just that scenero, it's call Gripit. They are bits that fit into a drill/screw gun, one end burnishes the stripped head, you flip the bit over, and the other end removes the stripped bolt, screw, etc. in reverse. Drilliing the head off is the next best option I think. I would then soak the remaining threads with PB Blaster, kroil, or another penetrant before trying to remove the remainder.