Rather than being buttfrustrated Clemente, you could actually post something related to the topic at hand, or not post at all.
(Also, talking like that to a moderator is just ASKING for punishment lmao)
Anyway,
Rosalina excels at mid-long range, but her close range game is considerably worse compared to the former, so if your fighter has a good ability to weave in and out of her danger range (and thus, bait her into unsafe options) or has armor on moves to tank an assault and deal ever-important damage to Luma, then you likely don't fare too bad versus her. I'm not implying she's helpless without Luma as others have already went over, but her onstage game and mid-range abilities really weaken without Luma by her side, and while not seemingly that long, 8 seconds is actually a decent amount of time in fights where fighters are in your face rather often.
Also, if you're partaking in a customs meta, most characters have access to "Piercing" projectiles or some other variation that Luma can't simply sponge, meaning she'll want to rely on GP more, and that move, while good, comes with some decent endlag and is very punishable if baited out.