Lots of release oil. there's a german one but I can't remember the name.
Is it a pozi/philips if so put the screw driver it it and give it a tap. Not a hard one just try to make it bounce no follow through a tap that lets it bounce back.
Then some more release oil and do it again. You are trying to break up the rust particles to let the oil in, not trying to break the bond in one go.
As it is cold out a bit of heat from a hair dryer or paint gun thingy might help in between taps. Oh and

while the release oil does its stuff, well that's my excuse.
If you need something done quickly just ask a busy woman then get the f"k out of her way.