I have some experiences against rosalina matchups. You want to mix up your approach with peach ground float with her neutral aerial. From there, you can start pressure with her down aerial and neutral aerial with spacing them. If luma is with rosalina, you can damage it with peach bomber and peach's tilt attacks. If you manage to hit rosalina on shield with peach bomber, you are safe and rosalina can 't counter back easily. Use this advantage to punish her with peach's neutral aerial with fast fall but don't abuse it too much if your opponent catches on. When you are recoverying high against her or offstage, don't challenge her moves and focus on getting back on the stage. Time your up special and recover low is your best option to recover. When you get rosalina offstage, use turnips to bait an air dodge and punish her with peach's neutral aerial or forward aerial while near the ledge. This is a matchup you have to take your time with since rosalina and luma has more range than peach has. Spacing peach's aerial and ground attacks and edgeguarding rosalina and luma is the key to winning this matchup. Good luck!