All right so there's the obvious things like make sure your punish game is on point. Jab to kill moves, up throw up air. Get those down and suddenly you're a lot scarier to the puff. Stopping your run with a crouch and jabbing is very scary, because your left over momentum pretty much always leaves you in a spot to upsmash without having to dash or anything unless they hard DI/SDI the jab away from you, but you can still get a guaranteed upsmash out of that. Spacing jabs makes your neutral and shield pressure game much scarier for the puff because there's not much she can do if you're spacing properly.
You were pretty decent at comboing with nair but that leaves puff horizontally with you and up throw up air forces her to be above you, and while she can spend a bunch of time above you stalling she still has to come down eventually and she's pretty weak when she lands because of her low priority beneath her.
Your neutral needs a lot of work. Your main strategy against puff was shoot the gun, which isn't necessarily a poor strategy but you want to shoot the gun to force puff to approach you through center stage and not to force her to approach you while you're by the ledge. Most of the time when you shot the gun you were actually cornering yourself, and the puff got a ton of mileage from that because your way of getting back center stage after that was go around on platforms or roll behind her. There were a couple of times when you'd roll behind her and then immediately run to the other corner and start lasering and let her corner you again. Try seeing if you can crouch cancel a forward air, wavedash out of shield to take back space, back up even further off the ledge to get her to whiff something, etc. Towards the end of the stadium game you started playing this a lot better and also walling her out once she got forced to the ledge, which is good and payed off well for you even though you lost the game still. This was pretty much because after you shot a few times you would nair her between her aerials, got some damage off of that, and then forced her to try to get through your back air. However, you can't just stay in once place and back air, because puff does have ways of getting through and hitting you out of it, but you can back air once or twice to force her to move in one direction and then punish that. Act like you're giving her some space back but being prepared to hit her when she tries to take that. Try to do that without overextending, and if she doesn't commit to taking the space or backs off soon after she tries to take it then it's safe to back up a little more and shoot the laser. Shoot it just once or twice, just enough to remind her that she always does have to approach you, but not enough to actually let her get substantial control of center stage.
You did all of this better on Yoshi's, except when you did laser you were either way too close or ended up leaving yourself in an awkward position (eg in your shield with your back to her). It's possible to laser on Yoshi's but it's hard to do it safely. It also felt like you were a lot more comfortable using the platforms to get in and threaten her when you didn't have control of the stage. On Yoshi's you would return to center stage using the top platform a lot, which is pretty difficult for Puff to actually be able to punish, while on Battlefield a lot of how you'd return to center stage would just be in a pretty predictable path over the side platform and usually fall down with an aerial after that, which is typically not safe if you whiff that close to someone. It's okay to not throw out any attacks and just take the stage back in that situation, the aerials should really only be used if she's trying to go in that direction as well. You can also go to the top platform again and use that as a way to force her to give up center stage. Puff usually has to overextend to get to the top platform so going up there while she's chasing you can generally give you a hole to get down and somewhere safe. You did this once or twice on Battlefield and you definitely got a kill from her overextending once.
Overall the match up is really broken in Fox's favor, but on Stadium and Battlefield you weren't playing enough to your strengths and were really putting yourself in some poor positions and getting out of them in a way that was pretty easy to punish. I think if you can iron those habits out you should be able to get sets off of this guy, because the game on Yoshi's showed that he wasn't really outclassing you and able to read your every approach, just his gameplan was countering yours on the other stages.
this combo guaranteed to make them realize their character isn't viable in this meta)
