take the seat off the post.
take the wheels off the frame
carry what's left over to your well secured bench vice and clamp the head of the post in the vice
using the frame as your lever rotate the frame around the post and when it breaks loose you can work it out.
The reality is that ammonia can clean out oxide but only if there is space for the ammonia to get to where it's needed.
I tried all of the suggested methods to remove one from a Kona frame I salvaged but only the brute force method worked.