Hand impact works well. I bought a cheap one from toolking.com. If you have a cheap torx socket that you dont mind messing up, you can hammer that in with an extention and use a ratchet. There is no impact action this way, but it can be done. A more crude way is to grind the head of the screw with a dremel, then remove the other 3 screws and remove the remaining stud with vice grips once you have the housing off. Also, make sure you have a magnet wand......the are your best friend.
Travis