The best way to loosen up this kind of thing is to use a thin penetrating oil designed to unfreeze rusted bolts. Juice the joint then tap the top of the screw with a brass hammer. Hit it with a propane torch and just heat it up ----you don't want to make it glow (or you will soften the metal). You want to heat it ,cool it, heat it, cool it---then try to break it with the screwdriver. If it won't break under normal force don't force it--this is what will bugger up the slot. The cycling of heat and cool will break the frozen threads----keep juicing it with penetrating oil. It works.
I have done this on many bolts that people thought was a lost cause on boat trailers.