I've done alot of research on this matchup, so I'll say a few things. If they've been said already, I'm just expanding further on them I guess.
At close range (Luma included), we lose to her badly. Luma functions as both a meat shield and has fantastic priority. However, Rosa's Usmash, Fsmash, dash attack, and side b are all punishable on shield, and if you grab her it lifts her into the air so Luma can't really hit you, so there's that.
At close range (no Luma), we generally win. We can play like we always do, running and gunning. Rosalina still has decent frame data, but suffers from lack of range. I like to rush her down as soon as Luma's gone, doing as much as I can in the 13 second period. Some good things to note:
-Dsmash and multi jab become unsafe on shield
-Her side b loses it's projectiles
-Down b can be baited out and punished hard from how unsafe it is.
At mid range (Luma included), we're tied. Rosalina would have to either sit and block our hits, let Luma take the hits, or absorb the hits with down b. Whatever she does, it's in our favor. And of course, without Luma it's even better for us. However, there are some things to note:
-She can outright steal Metal Blade with down b. It can limit her options with it in her hand, but she can also keep it for herself.
-Luma can absorb pellets, Fsmash, and Crash Bomb
-Crash Bomb explodes on contact, forcing Rosalina to either down b them or approach.
-Luma has 50 HP and can act always, except when he's in tumble.
For recovery, we must respect her anti air (Usmash and the dreaded Uairs) and her offensive air to air. Personally, I start off going low to snap the ledge and mix it up with high once she tries to start intercepting my up b with dair. Don't try to muscle your way on stage, because she can punish landings VERY well. Vice versa, she has a hard time landing on top of us (Uair, upwards Metal Blade, Usmash). As far as getting up from the ledge, I recommend neutral getup into shield. She doesn't have kill throw, so you don't have to worry about her grab too much. In the end though, sadly, it can be very tough getting back on stage against her, so take your time and stay safe.
For edgeguarding, Dair and Leaf Shield can give her a hard time because it can force out an air dodge or a Down B to absorb the projectile. Her Up b is also super punishable, having no hitbox whatsover.
Kill moves:
-Bair (90% - 100% at the ledge)
-Back throw (140% at the ledge)
-Dair (at least 70% for the meteor effect)
-Dtilt (170%)
-Fair (160% at the ledge)
-Usmash (110% uncharged)
-Utilt (70 - 80% sweet spot; 140 - 150% sour)
-Fsmash (70% fully charged; 110% at the ledge uncharged)
To sum it all up:
-Keep her at a distance and work on getting rid of Luma with your projectiles. Once he's gone, go get her.
-Respect her while Luma is present. Engage sparingly.
-DO NOT linger over her head. Her anti air is very deadly!
-Bait out her punishable moves at kill percent.
-Ban BF, Lylat, Halberd, Delfino. Take her to FD and Omegas or Duck Hunt.
Hope this helps!