penetrating oil, patience, gentle application of heat
and/or a few taps with a hammer to start cracks in the brittle rust..
more penetrating oil and more patience
and a few uggaduggas later it'll be removed.
Small propane torch or oxygen fuel will help or a breaker bar with a cheaper pipe on that. Just need yo hold it steady. PB blaster or the like will help loosen it up. An impact gun (air or electric) will help.