After a tourney this weekend, I'm starting to rethink the Peach ICs matchup a bit. It's definitely winnable, you just need to be... really creative. I got to play 15 tournament rounds against Xelic, TX's #1 Peach--arguably one of the top 3 Peaches in the country--and barely lost out against him.
First set was 3-2 for me, all IC-Peach victories.
Second set was 2-3 in his favor, again with IC victories.
Third set, same.
I was very angry and threw my controller after letting 1st place get away from me, but in retrospect I did a **** good job against him. This tournament gave me my first victorious set and I learned a lot of useful stuff about fighting Peach. I also nearly beat him with ICs on Brinstar (I played too aggressively :/ ) and actually maintained an advantage for most of our matches.
1) Catch turnips. They completely change the dynamic of Peach's approach against you. If you have them you can just use them to screw up Peach's float and make her land awkwardly, or hand it off to Nana (jump in the air and hit Z to hand it off) for a wavedashing approach. It also makes it easier to stop her turnip approaches if you can catch them consistently.
2) Twin Squall. When she starts hovering at the sweet-spot height -- just above your up-tilts and smashes, but low enough that her aerials will hit you before you can use your own -- move around a bit. Before she can hit you with an aerial.
3) Light shield. Oh God light shield.
4) Master--COMPLETELY MASTER--the ledge-chaingrab in case wobbling is banned. I was able to get completely perfect 0-death chaingrabs on Xelic with it at least once per match. Peaches like to stay near the edges of the level, so this will happen more often than with other characters. I also think she's the easiest to do this against. On that note, you'll also want to practice up-throw up-smash, since maxing out the edge-chaingrab renders d-throw up-smash unusable. The ledge-chaingrab can also be done on platforms, so if you somehow get a tech-chase onto one, use it there too. It will impress Nana.
5) When she lands on the top platforms, your u-air and f-airs are completely safe pokes. For Yoshi's and FoD, poke with full jump u-air. For Battlefield and Dreamland, use full jump u-air. She is unable to hit you with d-smash if you do this. On Yoshi's and FoD, your u-smash also has a good chance to beat/clink with her d-smash on the smaller platforms. Don't jump into her, just smash at her. If it's a clink, you're no worse off.
6) Her jab may be her most difficult to beat attack. It's too fast to punish on whiff and she'd have to be dumb as hell to let you walk up and f-smash her through it. Fortunately, there's a very useful de-sync for setting up Nana's blizzard, which will give you the chance to storm in with a grab, f-air, or smash.
7) Bait using de-synced charge smashes. Charge u-smashes and f-smashes with Nana and you can encourage Peach to attack with a foolish aerial in an attempt to punish, letting you intercept with n-air and u-air; she also may just float away complacently and not notice you leaping in with aerials. This trick works against a lot of people, actually.
8) Watch for turnip pulls. If you can get at a certain distance and predict when the pull will happen, you can punish with a grab. That right there should be a whole stock, since Peaches like to stay at the edge to pull turnips. See # 4
9) On the subject of turnips, you can wavedash d-smash through turnips; you will get hit but Nana will go through. When you're close to Peach and she's holding a turnip she may try to stop you, and wavedashing into the smash will often connect and knock her away while she doesn't DI.
10) Don't use your jab. She'll crouch cancel it. Every time. At later percents (probably 90+) you can f-tilt and her CCs will slam her into the ground. She might even slip off the level and d-air while she falls, giving you a good edge-guard setup. In fact... I just had a crazy idea for an edgeguard against her, but I won't share it until I can confirm it isn't completely ****ing stupid.
10a) You are allowed to jab her when she's holding a turnip. Just be careful that she doesn't throw it and force you to catch it, letting her CC d-smash anyhow.
10b) Your best aerial for preventing CC d-smash is f-air. Once she gets past around 20 percent (assuming you still have Nana with you), CC'ing it will ground her.
11) Go crazy when edgeguarding her. If she's floating low and keeping you from edgehogging, just leap off the level with a fast-falling forward-air. If you spike her, her vertical sucks and gets her killed. If you don't, she'll eat some extra damage and you can still recover with twin-squall hammer.
12) Her b-air is a very spammable move, but she has nothing that can go completely through your blizzard. De-sync lots of blizzards and it will keep her *** out of your face. Jumping blizzard helps you do a bit of extra damage but it's riskier; if she's DI'ing up she can come over your head.
The last thing you have to really remember: Peach is effing hard to punish. A lot of her moves have deceptively low ending lag which gives the illusion they can't be punished. Excluding her jab--which I think IS unpunishable unless she jabs deep into your shield, though that's really stupid--everything can be punished with something. What you want to do is FAKE your punishes. Dash in and sit outside her range, let her dodge or whiff a d-smash, then punish that instead.
This matchup is still stupidly hard since she can camp you like crazy. Your best bet if she's platform camping is to force her to the sides and try to get her really high where your u-airs and squalls can start reaching her.
That's all I have for now. I want to play this matchup more and more until I can beat every Peach in the country
Hell, why stop there? Every last god**** Peach in the world.
Edit:
Also, against Jiggs with SoPo, b-throw f-air is a really useful combo at mid-percents. I learned that awhile ago and just remembered to share.