Try taking a bit larger phillips head screwdriver than the hex opening and see if that won't start it out.
IF the bushing begins to screw out then just take it out and you can easier deal with the screw with it out of the gun, just let the grip panel push outward as the bushing comes out, once the grip bushing is out then you can tkae the panel off the screw/bushing and then deal with the stripped screw.