Copying from the thread...
Yonder said:
Tornado is great in this matchup, it goes right through Luma and tosses it aside, even killing it sometimes. I like to focus on killing Luma first, then attacking Rosalina after because she doesn't have much of a chance against Luigi without Luma, super easy to combo and D throw to Up B, D throw to tornado is nice too because she'll most likely be stalling up there. She can just run away though due to faster mobility so that's a bit of a problem. Fireball is more useful in the air than the ground/ With Luma in play, it's worthless on the ground.
One Tilt said:
Definitely ignore Rosalina whenever Luma's up if the game's in neutral-- our approach is limited enough without trying to rush through that kind of obstacle. Fireball actually fires quickly enough that it's rarely efficient for Rosalina to down B them all, and, if she's spamming that, she's not attacking, and you're still potentially chipping Luma with a few fireballs. The key seems to be in out-camping Rosalina, as the most Luma might do is fire off some star bits so long as you keep out of its attack ranges and don't walk into Luma shots. Once Luma's down, just keep in mind that you need to take advantage of the speed of your aerials, as that's the key advantage you have over Rosalina's powerful disjointed ones. Even if Luma's up, it offers very little advantage in the air compared to the ground. As a note, while Rosalina's recovery is very quick and it has some great range, it's still quite feasible to stage-spike her. Given her own very slow air speed, though, it's more difficult for her to gimp us with her predictable Dair or such-- though, I normally see Rosalina's edgeguarding instead of hopping offstage to intercept lower recoveries (though that Fair plus Luma shot/star bits is quite the threat horizontally, so be wary of recovering too high). That all said, the match seems to depend on staying out of middle range. At long range, fireball forces her closer, while our only midrange tool is pretty much tornado-- which is unsafe on shield. Up close, our grab and quick air moves give us an edge, but butterfeet makes it difficult to 'stay in' on the ground for long... It seems like a battle of attrition at times, but we have a bit of an advantage in KO power, given the slight difference in weight and our myriad options for earlier KOs. Though, it's pretty difficult to deal with Luma combos if she manages to play aggro enough without making too many unsafe moves... if you can disrupt her control of the game's pace, you'll stand a much better chance.