-Peach's best tool is the match up is crouch cancel down smash. It sets ups for dash attack tech chases, which can lead into chain grabs, more dash attacks, or aerials combos. To avoid letting her cc dsmash you, you can't have predictable short hop nair approaches.
-Fox's dair cannot be crouch cancelled, and leads into shine if she does not sdi.
-Obviously, abusing waveshine>upsmash/nair/grab/dsmash is a necessity in this mu, given how relatively easy it is to waveshine peach. When finishing off waveshine combos, it will benefit you to learn how to read her DI and mix up accordingly. If you waveshine her all the way to the edge, essentially you can mix up between nair and usmash. Survival DI for usmash will send her at the worst angle for nair, where as DI'ing so you can recover after a nair will mean terrible survival for the upsmash.
-downsmash to finish a waveshine chain is good as well, but you can DI for the upsmash and option select the amsah tech in case they down smash, so if the peach is knowledgable it will likely not be a good option compared to nair/usmash.
-Do not let her pull turnips safely. If she tries to pull a turnip when you are close enough to punish her for it, you better get the punish. Otherwise, you need to stay in close quarters so she never gets the opportunity to pull a turnip.
-If she is on a platform, don't immediately jump into her and try to pressure her, because if she knows what she is doing she will atleast get a downsmash trade every time (a trade which she wins). Instead, wait for her to commit to something and then react to her decision