All of Peach's aerials have good priority but are beaten by the range of Marth's fair. Just learn to space it properly. Peach also has trouble making it back to the ground if you were the one who got her up in the air in the first place. Just utilt if she tries to land near you and tipped fair if she tries to get away. If Peach is on the ground, don't miss or you'll be punished with a grab or dsmash. Dashdance a little to try to get an opportunity for approach (like when she tries to pull a turnip). If you get grabbed, you probably won't get uthrown. Her fthrow is relatively quick, but her others are slow, so if you don't have particularly good reflexes, DI expecting an fthrow until you see what it really is. DI Peach's dthrow behind her because otherwise you'll pop straight up, which sets up into a nair or sometimes even a usmash. If you're facing a campy Peach using lots of turnips, try to swat the turnips away with fairs. Don't do this to all of them or else they'll see it coming and just hit you with an aerial during the lag after the fair. If you're against a slightly aggressive Peach who isn't very familiar with the Marth matchup, just keep spacing away with fairs, and if any of them hit go for a combo before she makes it back to the ground.
The game against Peach is all about taking full advantage of Marth's range! Peach can't get close to Marth if he doesn't make any mistakes, but she can deal a ton of damage the moment Marth does.
