Play Fox lel
For real though the matchup is real hard. You have to be incredibly patient. We have 2 moves at our disposal that outrange Puff's bair: turnip and fsmash. Abuse that fact.
Basically, you have to stay grounded. If the puff actually knows what to do, all they will do is space bairs. So, you've gotta wavedash around and fsmash a lot. Pull a turnip whenever you can because you can do like turnip into fair with can kill. The hardest part of this matchup imo is killing the buff. Obviously there is stichface. Sweet-spotted golf club as well, and depending on the stage, possibly tennis racket. Turnip -> fair will kill at a decent % like around 130 or maybe later depending on the stage. Nair might also kill too, not sure what % tho, again killing puff is all VERY stage dependent. Like if you take a puff to dreamland first off if the puff player doesn't start laughing his ass off he is mean lol and second off accept that you're not gonna kill the puff before like 200. Don't take puff to dreamland lol. Anyway bair might also kill, also FC bair is a good OOS option if puff is pressuring from behind. FC or DJC nair OOS would be good if puff is above or pressuring from front.
Also try out dsmash and see if it works on the puff. If it does, use it a lot, if not don't use it. Sometimes it ****s over puff if the puff doesn't know what to do about it but if the puff does then you're gonna rested and that is a bad time.
Anyway I hope this helps. I would honestly recommend picking up Fox for this matchup if you do eventually want to get to the highest level of play. It's just too bad.