I say spray the edges with plenty of WD40, forget it for 7 days and hit it with a block of wood and observe carefully if it has moved.
Even better would be an air hammer that I have used to do stuff a hand one couldn't.
It uses powerful pulses instead of brute force and has gotten me out of trouble before ( not for this though).

Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!