Heat and vibration are your friends.

I spent 20+ years working on the rustiest salt-damaged Pennsylvania vehicles known to exist, Volkswagens. Before attempting to twist the cup from the frame, it helps to fracture the corrosive bond by hitting the cup with a hammer. Not just once or twice, but maybe 100 solid hard hits. Applying heat to the bottom bracket shell will also expand the shell and break the bond of any corrosion or Locktite. One should take precautions with the finish if saving the part is a concern.

The vibration from an air chisel will also loosen tight parts. Chisel the part in the direction it needs to loosen. The chisel will dig in and mark the part, so that is a concern if the part is to be saved.