If you cannot get a small pair of grips on the outside of the screw and unscrew it that way then you are going to either have to drill out the screw, which will then mean retapping the thread, or see if you can get a self tapping screw into the top of the stuck screw and try to start removing it that way. You can either screw the self tapper in then unscrew it and hope it starts the thread on the stuck one, or you can glue it in place and then unscrew both together.