Boiling water is a great idea as it won't harm the paint badly like a heat gun or torch might.
I had one that somebody had cut the top of the seatpost off. Aluminmun in a steel frame. No effective way to grab onto it. I took an old bent frame and drilled a hole in the Head Tube. Then driller a hole in the stuck seattube. Bolted together and twisted away. Broke the bond that ammonia or PB couldn't get too.