DuskMarthWater
Dude.
Yeah, empty hopping can be useful too. But honestly, punishing floaties above you is really a test of how good you are at the baiting and waiting game; the skilled ones are looking for you to rush up there and smack them with an aerial, believe me. Sometimes that will work and they'll take the hit, other times they'll up b or float away and be tricky with their jumps, or they'll air dodge (Peach has obnoxious floating tricks too), and often if you don't play patiently enough they will find a chance to counterattack you before you get your punish off. You can't be so willing to punish that you put yourself in a bad situation. So baiting and watching them, looking for an opening, is what you should practice. It comes with patience and time; being able to force your opponent to do what you want them to (even when they think they have a selection of many options, they really all can be reacted to and lead to the same conclusion) is a very difficult skill to master.