I heavily disagree with the picking of Battlefield against Rosalina. Anything with a platform where she can have an easier access to juggling you with uair is bad news.
After a series of friendlies and a MM I feel that for this matchup it'd be better to utilize full hops, try and use mixups with it or just spaced full hop fair to avoid getting caught up with Luma things. If you're feeling shorthops then use your nair, spaced tip on shield and follow it up if the opponent buckles.
Keep in mind, despite being a giant, Rosaluma is incredibly light. Allowing us to net a kill with Upthrow around 167% if you find yourself lacking the ability to get her off stage for the kill. Which brings up point 2. If you're going to win this match up, YOU MUST make every opportunity you get Rosalina off stage count. If you find her positioning herself for the UP B instead of trying to get back to the stage as fast as she can, ready yourself to swat her away with your aerials. If she doesn't position herself for the ledge snap, down tilt from the stage and use your best next action off stage. Don't let her get back for free, the match is uphill enough as is. If you're netting grabs you better do a pummel and throw or else Luma cheese will get you. By doing this you've quickly separated Rosalina from her Luma and can deal with the Luma quickly when near the edge.
She can beat you up off stage as well, but don't give her what she wants. Mix up your recoveries if you see the bair coming don't hit the deer in the headlights strat and just take it.
Things that did not help as much as I thought it would: ShieldBreaker, as always just use this sparingly, the reward for a little bit of damage to Luma isn't all too great when your end lag is that massive.
I'm guilty of this, but the first step is to respect Rosalina's kit when it's just her as well. Don't get caught taking free damage by things like her dash attack, that has two hits to it not just one.....so shield it like it does as for this will be a great move to punish since it seems to be the option of choice when Rosaluma players lose their trusty sidekick.
Personally I'm not a fan of the matchup, nor do I have enough experience to really give it a number. But as far as notes this is the best I can do. Hope it helps.
In Short:
No stages with high platforms, unless you're a masochistic badass.
Get Rosalina off stage and keep your damage and kills that way
Don't pummel too much on grabs, bad
Be on the lookout for dash attacks from Rosalina when it's just her, punish accordingly
Full Hop aerials or spaced nairs are you friend