This will rather depend on the quality of the screw and the case, and how much you're willing to hammer on both, but:
You'll need a small punch, a hammer, and an electric screwdriver equipped with a hex bit of the correct size. Do not try to do this with a "normal" allen key.
Use the hammer and punch to bring the hex hole back into shape, by hammering over the "flowered" bits of the head, toward the hole. Don't worry if it looks a bit ugly.
Take the driver bit, and force it into the hole. You should need to force it, preferably by hammering. If not, continue "reducing" the hole with the punch. Once it's in, and /tight/, carefully attach the driver bit to the electric screwdriver, and, on maximum torque and *straight away* on full speed, unscrew the screw.
This works very well for "mechanical" parts (think corroded steel M5 hex bolts in aluminium blocks attached with threadlock and corrosion), but the hammering side of things may reduce its utility on keyboards.