I had this happen once, I took my dremmel tool and cut a new place for the screwdriver to go in, it does'nt have to be perfect, just enough for it to fit. Then I have a small impact screw driver that I place in the new slot and hit the end of the screwdriver and it unscrews the old screw kinda just like a extractor, or just use a screwdriver. Oh and next time put some never seize or BP Grease on the screws.
Good Luck
(BP)