Your trying to use Peach the way she was used in Melee. That be bad :x Save the F-air for KOing and use B-air and D-air for approaching and damage. Peach's Up Smash will KO at 80% when sweet spotted at the shoulder pad unstaled.
D-air combos into N-air when you stay floating during it. If you fast fall it to prevent the final hit it can combo into a grounded attack so long as its a quick opener. Use the U-tilt as an anti-air as it has a pretty big hit box.
My Creations in Modnation Racers! Check zhem!
http://community.modnation.com/en-us/players/IvoYaridovich/creations