@chanadler this will totally depend on what the srewhead looks like right now. Sometimes a larger size screwdriver will have a bit more bite on it. If that does not work, I'd drill it out. Use a hand drill (not a powered one since it could pull in to deep) and a bit as large as the screwhead. This will ultimately take the head off but leave the threaded part of the screw in. Once you remove the case, use a pair of vice grip like pliers or a good pair of hemostats and remove the rest of the screw.