When using manual screwdrivers to loosen a screw, sometimes you need extra leverage to get the screw started to bust it loose.
If you have a square-shaft screwdriver, all you need to help you loosen the screw is a small adjustable wrench. Just put it on the shaft, tighten it up, press down with one hand really hard and then turn the wrench. Once you get it started, it loosens up enough that you can pull the screw the rest of the way.
If the screwdriver has a hexagonal shaft, get a small ratcheting box-end wrench, slip it on and again use the wrench as leverage.
If you have a round-shaft screwdriver, get a pair of locking pliers. Just lock them on and out comes the screw!