Peach Sheik in NTSC...
Make her get into the air and slap or bair her appropriately. Needles are good for making her go airborne. After you slap or bair her, your follow (assuming you get a follow) depends mostly on positioning and stuff. I find bair > bair is pretty common at low percents, and fair > f-tilt > fair is pretty common too. Fair > dash attack > ??? becomes pretty good at mid percents.
You can f-tilt through her aerials too, if you want to. You have to do it proactively rather than reactively, though, so be careful with it. You can also dash attack under her aerials (not dair) if you time it and position it correctly. Takes some getting used to, though.
If Peach decides to not approach when floating, remember that her float only lasts so long. You can honestly just sit back and charge ground needles and throw them at her when she's coming down if she's trying to be hyper conservative. Just be prepared to cancel the charge and swing at her (or retreat) if she starts coming at you. Her only option at that point is to attack (which is fine), try to throw you off with garbage like air-dodge that doesn't do anything about needles, or retreat to a platform. Peach being on a platform is a good position for you as long as you're not stupid (just don't jump into down smash).
When blocking, you can't counterattack after FC aerials. You can counterattack after dash attack, jabs, and down smashes. And anything that's spaced badly (f-smash, badly spaced d-tilt, etc).
If she decides to play a ground game, it's not too different, only you're supposed to play a bit more defensively because attacking recklessly into Peach's ground game gives her more chances to get under you. Use a bit more bair. WD back, shield, dash dance, etc. Just flaunt how your movement is much better than her's and you'll be fine.
Protip: When you're spacing bairs, if she comes in for a dash attack, instead of bairing just triangle jump away from her and then punish appropriately. I like to turn around and grab, but if f-tilt tickles your fancy and it's at a percent where it'll work, then go for it.
She's gonna crouch a lot so you have to be good at aiming your aerials relatively low to the ground (while still retaining the AC in the case of fair).
Lightshielding near the edge to force yourself to grab the edge is massively underrated and very good because she can't actually do that much about your poofstall without being kind of obvious about it. I mean, if you just poofstall over and over again she can eventually steal the edge, but you can see her getting that position and switch it up (baby ledgedash is amazing). If you can catch a turnip, you can make it really hard for her to cover all your recovery options onto the stage even if you forgo the use of the ledgedash so keep that in mind.
If they're really campy, if possible, push them to the edge (get them trapped under a platform, etc). From there you can just pressure them and zone them until you hit them offstage and then do some systematic aerial & tilt algorithm and you'll cover just about everything. You shouldn't concern yourself too much if she grabs the edge because you can cover most of her getup options really easily because she doesn't have a proper ledgehop (no ledgedash, really slow ledgehop aerial, loses almost all invincibility if she tries something funny, etc). Needles are amazing in this position.
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