I'd recommend some heat to shock it (plumber's freezer spray first, or even better a couple of hours in a deep freeze - then boiling water over it).
Then a week of plus gas - a squirt in the morning and evening.
:Finally: mount in a vice and undo.
As said, don't go mad. Firm, controlled, increasing pressure and no lugging it about. IF anything looks like it's slipping, STOP tighten and re-try.