You need a really big screwdriver to remove that sucker without damaging it. I think the official tool is a "#10 screwdriver", or the screwdriver end of a DPMS combination armorer's tool/barrel wrench. You can probably use a 1/2" or 5/8" wide standard screwdriver, too.
If you've mangled it already, order up a spare. In a pinch, you could drill it and use a broken screw extractor kit from most any auto parts store.