When a screw is rusted and it has a damaged head try soaking the screw in some WD40 and then use a hammer with the screwdriver inside the screw.
Hit the screwdriver as sometimes that will loosen the screw enough to allow it to come out but if that doesn't work then you may need to grind it out if possible and drill the screw out.
You can also go to a hardware store and purchase a damaged screw remover that removes stripped screw heads.