Knock them out from below using a long rod or pipe. Just get the pipe pushing against the bottom of the piston crown and then pop it hard with a 5 lb sledge and the piston top will break off. Only takes a couple of seconds once you get the hang of it.
If you have cast pistons, this way works well. When I was a kid and made money selling scrap cast, every junk dealer would dock you half the weight because of the aluminum pistons. Removed literally dozens of pistons this way and I can't remember the block being damaged on any of them. Never encountered forged pistons but I would bet you couldn't use this method on them.
