If she is on the ground, she only has 3 options. Dash attack, Shield, Pull turnip. To beat this, just pressure her with dd grab, make sure you shield when she approaches and shield grab that *****'s dash attack. A lot of the ground game is simply keeping her from being able to pull a turnip. You want to be close enough to WD/dash forward dtilt/fsmash on reaction to her starting to pull a turnip.
If she did get a turnip, she gains a big boost. Her shield game is much better, and she has some real approach and defensive options. Most likely if you give her some room, she will take to the air. Make sure when she does you are on her ***. If you nair her as she throws the turnip, you will hit the turnip (protecting yourself) at the same time you hit her. If you're not close enough for that, you're going to have to bait her into closing the ground. Most likely you're gonna have to shield a fair, and prepare to roll. Its one of your safest options against dsmash/shield pressure happy peaches. Also peach is pretty slow so its hard to cover the rolls. Once you have her back on the ground make sure she doesn't get another turnip.
Basically, you can outcamp peach as marth as long as you're semi-decent with grabbing turnips out of the air and powershielding them, ect. But you don't want to. Peach loses to a marth that stays (safely) on her *** and gives her no options. Marth can actually systematically edgeguard peach, though its a weird method (thx tec0).
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