I haven't fought any competent Rosalumas (I've fought some terrible ones though
) but based on my own playstyle and her tools I hope these might help:
1) if she has her luma attached to her (she hasn't shot it out at you, or she's summoned it back to herself with her twirling wand) then she's pretty much guaranteed to blast you with luma if you create any distance and are on the level with each other, so watch out for it when you approach. Not all rosas will charge their luma to fire it so you want to specifically watch around the distance an uncharged luma launch would reach you.
2) her dashing attack is incredibly meaty/long in duration and adds a whole bunch of reach when it goes off. I'm also somewhat convinced that during the dip she does before she comes up and headbutts you it might also low-profile so it could potentially go under a lot of things. I've not tested that though so take it with a grain of salt... I just noticed it beat a lot of things I didn't expect it to
Just be careful when she runs at you and if you do block, hold on a little bit longer than you'd expect before relinquishing that shield button.
3) Losing Luma isn't as big a deal for a Roz player as many would think. Her normals are pretty darn good so I wouldn't get too hung up on taking out Luma. Instead, just try to keep out of smash/star-bit range from it and you should be ok. Again, this only applies if luma has been sent out. If Luma is attached to Roz then expect it to be shot at you instead
It might seem like a lot of effort to keep track of Luma while also fighting Roz but keep in mind your opponent's attention will also be split in the same manner if they hope to actually make use of the little star. Despite some claims to the contrary, Roz is the biggest source of damage for this pairing. While Luma is safer for Roz, it does less % and knockback than it's mother (I tested in training
) Also, Roz does NOT get a damage boost for having Luma attached, any additional damage she gets is just from Luma also attacking at the same time and hitting (again, I tested in training as I was curious :3 )
4) Coming at Roz from above or below is pretty dangerous. Her air up and down are both blessed with good reach and disjointed hitboxes (it basically sends out a hitbox, creating a fairly large buffer between that and her hurtbox.) If you have to attack her from the air it's always safer to come in from the sides.
5) If you ARE edge-guarding Roz, keep an eye on Luma. You have no idea how many KOs I've got from someone focussed on me making my way back and not noticing the little star bouncing up behind them giving me the perfect position for a Smash KO.
I feel like you probably need better info than this but again I've not had the privileged of facing a good one myself so I'm just summarising what I do when I play her and the things I've noticed fighting the lesser Rosas :3
edit: Actually, I guess I should explain how Luma moves when seperated as that might help both opponents and those actually playing Rosa. Basically if not given any commands, Luma will automatically move to a position a set distance infront of where Rosalina is facing. This means if Rosa turns around and waits, Luma will run past her to a point at the set distance in that direction. This also means that when rosa sends out luma, if she doesn't give it any attack commands for a bit, it will retreat back towards her until it reaches the set distance.
I've still to work out how the heck the little star works when Rosa is tumbling from above though, it jumps to try and reach her height but it's not as set a pattern as I'd like so I've often had it jump as I'm trying a smash or tilt and had it whiff over my opponent's head XD
Anyhoo, hope something in all those ramblings was helpful to someone