All suggestions so far are good; I'll add another.
If you can see the piston skirt through the exhaust or intake port, put a big punch through the porthole and start hammering. If you are lucky, you might be able to collapse (buckle) the piston skirt, and that should help free it up. This assumes that the piston is seized to the bore, in which case it's already a loss and busting the piston won't hurt things any further.
By your comments about crowbars etc, I take it that you already assume the piston/cyl are a loss.