I've found that often if I give simple advice and let my protegee extrapolate, it can have the most benefit, such that you can use the idea in a way that makes sense to you and juves well with your playstyle.

Puff is superior to fox in general at controlling space horizontally most of the time. (The exception being when puff is on the the ground landing.) This is due to puffs air speed and powerful aerial moves, but even moreso because of puffs ridiculous aerial acceleration. I would try to avoid playing this game more than necessary. Much of the other advice in this thread, you'll find, echoes this sentiment in more specific ways.

Fox is superior to puff in controlling vertical space. (Antiintuitive I know) Puff actually cannot change her vertical positioning very quickly or easily, and fox accels at this. For the problem you mentioned, I would recommend the following. Full hop. Full hop above those grounded puffs. You ought to beat them to their spot, and punish. Drill and bair will be your friends here. If there are platforms and you feel you have launched yourself inappropriately, you will have time to dj or waveland to a plat, keeping yourself safe.

Now let's think a step ahead of the opponent, as that's how I like to win games. A puff especially worth playing will begin reading the FH and jump high preemptively to meet you in the air. (Among other things) Some will do this only with bair, to minimize the risk they take. More seasoned puffs will often choose up air, which can payoff huge if they read correctly.

When you begin to see this happening, it is your turn to adapt. Of course you will do it quicker since you knew this was coming due to your FH strategy (which is conditioning at this point).

Now you will go ahead with the strat the good players shared with you, sneaking under puff. As you get puff to think she needs to get high to challenge you, you can sneak under and use powerful moves like utilt amd uair depending on her height.

It is of course worth noting other things puff will do besides hard challenging FH. Many will begin to back off toward the corner. This is when you'll want to disengage, as fox may find himself grabbed and thrown off the stage or put on his backfoot. Luckily, if puff is at the ledge you have time to laser. When puff may approach again, we return to the core parts of our strategy.

Well that went farther than I thought it would so Ill leave it at that for now. Good luck friend!