It wasn't on the same screw your dealing with, but with soft metal, I usually provide some support to the opposite side, then I use a real sharp chisel the correct width to re-groove the screw head. Then once the chisel has cut into the screw head pretty deep, use it as your driver. Just make certain you support the opposite side of the screw against something solid, maybe center a ball bearing against the opposite side, put a tiny amount of super glue on the ball bearing to hold it in place so it can be solidly supported against the anvil, other wise you'll severely damage the threads.
You can also try grooving it to accept a phillips head screw driver, that would provide a little more support while trying to loosen it.
This method may not work if you have no way of supporting the opposite end of the screw.
GS