Lots of penetrating oil to top and the next day to the bottom of the seat post by taking the cranks and BB off/out. Move the seat as far forward on the rails as possible and use the saddle nose to apply torque.
You can apply a lot of torque this way. Next step after a week or two of repeated soakings and trials would be to clamp the seat in something, a slot the width or a bit less of the seat with the bike upside down and use the frame to torque the seat. Either the seat clamp will give/break or the post will loosen and you will be able to wiggle it off. If the clamp breaks then it gets complicated. Is the post steel also?
If the saddle rails break or come loose cheap saddles are probably available throwaway at the LBS for further efforts. Steve