Kaze's got the right idea.
turnip camping becomes relatively ineffective, and though you can dash attack marth, you can't kill a well spaced marth on FD until after he kills you %age-wise. Dash attack and turnips will not kill marth, and those about the only moves you will be hitting a good marth with on this stage.
And excepting the turnip drop trick, i've come across very few consistent ways to edgeguard marth.
Good news is peach should be able to recover against marth, she's got more than enough options of recovery to bait out his idea and move past it.
Platforms are peach's friend because without them she never gets any follow ups to any move (except an approaching turnip -> aerial (which should get naired through anyways), and dash attack to dash attack at low %'s). On platforms you can dash attack marth jump up to the platform and dsmash his tech, or uair through it, as well as other options. Peach can also wrap around platforms with DJC to catch marth off guard. Plus she can use them as different ways of getting back to the ground, whereas on FD you have to hope you airdodge his uair AND land back on the ground before he can grab or utilt you. If you're getting tippered through platforms, stop landing on them above marth... Tippers from platforms shouldn't kill peach until she's at a high enough percent to where she doesn't have to tech or can pick the platform she's going to land on. Seriously, just jump when you land on a platform.
It IS counterintuitive. You would think FD would be good for peach, but it's not. Against Marth anyways. But go with what works for you, this is just what I observe at the highest levels of play. As well as my own experience (which isn't that far off from the highest levels of play if i'm flattering myself.)
Also I observe Marth dominating EVERYONE on FD. Except CF.
It's great for peach against other characters though.
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Try to smash DI out of CF's nair combos.