Ganondorf is another matchup where peach needs to play patient and especially campy but smart turnip game, but then beat the **** outta ganondorf when he's open to combos. If peach just rushes in, ganondorf has easy counters to that stuff, namely just kicking you away or fairing or aerialing you. Ganny is tall so has tall shield, so you gotta be precise in float cancels and especially not to let ganondorf to grab you. If he gets a grab and dthrows, but misses his timing a frame or couple, you can nair him before he does his stuff, but don't count on it. When float canceling, you can get his feet with dsmashes since he has long legs, but be careful. Watch out for ftilt, especially upwards directed. When following up in combos, beware of his dair. Uairs, dash attacks, dtilts, ftilts... whatever you come up with, can combo ganondorf (I believe exarch has a mini-combo video about combos vs ganondorf in da internet). For recovering back, watch out for those aerials. Uair has reach and peach can't for the life of her overcome that move. Fair is super deadly, don't get hit lol and bair is fast and killer. If you sweetspot, prepare to ledgetech. To maul ganondorf's recovery, turnips are a good way. Throw down at his head to make em mess "sweetspotting", but dair is usually a safe maneuver to get damage and make him mess up at some point. My personal favourite against good walljumptechers is to space a fair, then immediately run away (or just spacing) and nail em with a nair when they try to come towards stage. This also works against c.falcon (even better). Dtilt is fun if you get it, as is tennis racket. For OoS defences, nair oos is a good one if you get it done right. Wavedash to stuff if you're quick like armada lol.
In short, use smart turnip camping and don't let ganny-uncle to hit you or block your turnips with his own hits (<-this!!). If you get him into a combo, improvise, quite many of peach's stuff combos ganon (just don't try to fc quick nair in succession at low%, he'll cc that and punish/escape...). Space extra well, he has more reach so you'll need some timing and spacing to get at him especially without turnips. Beware of his ledgeguards, normal and uair spikes too. Watch out for random wavelands. Don't get grabbed.
For video reference, imo armada's pool matches against Fear is how to mess up against ganon when you don't perform quite right, but 2 next vids are how to **** ganon and how to do it right.