The last stuck seatpost I removed was also the worse I'd ever had. I kept the frame inverted and filled the seat tube from the bottom bracket with Kroil, to a level above the post. I checked it at intervals and ended up soaking it for a few months. I clamped the seatpost in a very large vice that is bolted to a heavy workbench and eventually hard twisting back and forth caused the post to break loose. It still took a lot of soaking and twisting to get it all the way out. Caustic soda would have been more satisfying and faster, but I wanted to preserve the paint, if possible.
I spent years wrenching in bike shops and this was not the type of job we liked to take in, especially during the busy season. If the post wouldn't move after a reasonable amount of soaking and twisting the saddle, we'd return the bike to the customer and tell them to bring it back in the fall, which they never did. The trick is to avoid wrecking the post in that initial attempt.
Never wail on a stuck post with a hammer. If anything, it will wedge the post in even harder. When your arm gets tired, which will happen quickly with a 3 lb sledge, you'll likely hate yourself when the hammer glances off and dents the top tube. The hacksaw method is far more difficult than it would seem to be, even with only the minimum amount of post in the frame (would you be so lucky). We once had a bike come in where the guy had cut off the post, slipped a bent rod through it, hooked it on the bottom of the post, and proceeded to apply enough force that the rod punched a hole through the seat tube. If you can't move the post by twisting, you're never going to get it to slide.