Neutral game vs Marth:
Some facts:
- he is a lot more mobile than you are.
- but he doesn't want to run away too much, because he doesn't have a projectile.
- his air to ground interaction isn't good.
- He isn't good at pushing into his opponents super hard, because his attacks vs shield game is okay, but not desirable. He can space aerials and get away with it, but it's not really all that easy to push from hitting shields into getting an opening like we, spacies or CF can do.
The conclusion:
He wants to get somewhat close, but doesn't want to close the remaining distance preemptively, but instead wait for you to do something. Which means, he'll usually dash dance there and wait for your reaction.
Your reaction should not consist of "get scared, throw out a downsmash (or spotdodge), get punished for it"
, but instead you are usually going to want to not directly attack from there, but rather do something to break his desired spacing.
Most of the time you do that by starting to dash towards him, which he can answer by:
1) use f smash or something else from his grounded moveset with a long range, but fairly lengthy wind-down.
2) use an aerial
3) move away
4) shield
you can do the following:
a) dash attack
b) use an aerial
c) actually pull back instead of performing the attack
d) grab
Well, it basically comes down to reading what he is doing. In general you want to dash attack his aerials, pull back on his laggy ground moves and punish afterwards, move back a little if he does and grab a turnip, and grab or aerial his shield. Well, it's not entirely clear cut, since we're talking about melee and not RPS (his aerials don't win every single time, dash attack can catch evasive movement because it's really good, etcpp).
He does have a decent advantage in neutral, but Peach's punishment game really evens out the matchup. He tends to have real trouble finishing you off, and you on the other side really bring the pain every time you get your hands on him.
edit: moving back is really good, because it's so non-comitting. Do a lot of these until he stops doing 1)
and of course there's a lot more to say about the matchup, but I think the above situation is the quintessential part Peach has to understand in order to do okay vs Marth