First, turn the tool end of the driver counterclockwise so it doesn't move when you turn the driver to loosen.
Second, hold some ccw tension on the driver when you smack it.
Third, usually works better with a small smack from a big hammer rather than a big smack from a small hammer.
Fourth, if the screw head is too boogerd up, you can sometimes hacksaw a slot in it and use a straight bit instead of phillips.
Other option is to try to grab the screw head with a small vice grip to loosen.
Or you can drill the head of the screw off and vice grip it out after the cover is off.
Fun, huh?