Start off the match with a laser immediately - 9 times out of 10 the first thing a Peach player does when a match starts is pull a Turnip (because having a Turnip in hand helps us approach) but, at the beginning of a match, the Turnip pull animation takes longer to finish than it does for ROB's laser to hit her. Every bit of damage helps right?
My advice would be to play really campy - ROB's quite a big target which sets him up nicely for Dairs -> Fair/Bair combos even at higher percents so you want to avoid Peach getting in close and personal as much as possible. Spaced F Tilts and Fairs are good for tackling her Fair. Defensive Nairs can also help stop her getting in. Her Fair has good range but your Fair outspeeds hers - you can beat her to punch if you think she's going to try and SH/Floating Fair you. Watch out for mix ups with F Tilt and F Smash though! Her Bair has a deceptive hitbox too and can punish any whiffed Fairs. ROB is quite tall so SH Bairs for Peach are quite a good mix up option for her.
Your Up Smash is strong - if Peach gets a bit aerial/Float happy you can catch her out with an Up Smash if her spacing gets sloppy. The window for landing her aerials laglessly is much less forgiving this time around so be sure to punish any poorly spaced aerials with grabs and F Tilts/D Smashes.
Really chase after her when she's trying to land and watch where she goes - Peach's biggest weakness is arguably trying to land after being hit up in the air. Likewise, her Uair is very disjointed and can beat out ROB's falling Nairs if he tries to use them directly above her.
Watch when you throw your Gyro out! Whilst Floating, Peach can perform aerials whilst keeping hold of any items in her hand and the same goes for the Gyro, meaning you won't be able to pull out another one until she lets go of it or you knock it out of her hand.
Hopefully those tidbits are useful - I would say the most important thing is to in this MU is avoid letting Peach get in close and personal too often by playing defensively, spacing well and using your projectiles in situations to throw the Peach player off their game.