Well, who knows what type of thread-lock may have been applied to the screws when they were originally installed? Someone could have used anything from Red locktite (strongest) to (heaven forbid) epoxy!!! I spose you should try a mild approach to getting them to loosen up i.e. soak overnight with a couple drops of penetrating fluid. For sure, use the correct sided screwdriver that fills the screw slot completely. After soaking over night, gently tap the screw head several times with a brass hmr or flathead punch (similiar to rapping on the edge of a jar lid that is extremely tight). If they still don't budge, then I'd take soldering gun and use the tip of the gun to heat up the screw. The heat may melt or soften up whatever was applied to the threads to keep them from loosing up. Good Luck.