You have 3 options that I can think of. We have a screw extractor set or screw extracting pliers, you can drill the screw out (not pretty), or you can dremel a slot into the screw head and use a flat head screwdriver (also not pretty). Any of these three methods will work fine, but keep in mind that the dremel and drill will put the phone through considerable vibration and a slip could mean more damaged parts.

Hope this helps, Good Luck!