Here is the trick:
When they just will not move, or are unaccessable by "hammer phillips"
go out and buy an ignition wrench plier from Sears. It is about $6,00usd
measures about 5 inches in length.
With this small adjustable wrench you can grab onto the screwheads and
need to merely move said screw a tiny bit to break it loose.
Then use conventional srewdriver to back them out the rest of the way.
This is the only tool I carry with me besides duct tape.
good luck,
michel