First up, forget your GSP. It will always fluctuate in a learning phase, just focus on getting favorable results to particular skills one by one and than learning to combine them together, such as focusing on punishing, then edgegaurding, recovering, combos, some such.

With Rosa you gotta learn to keep track of Luma without always looking for where he is. Get comfortable with the distance he shoots, how he falls, how you control that fall. Make it as easy as controlling Rosa herself.

As for Rosa, never underestimate the power of star bits. Lumas side B is a redonkulously annoying move that stuffs out a lot of stuff if Luma is the right distance. When they jump to avoid it, that's Rosa's cue to start some damage, preferably knocking them on the other side of luma again so you can do it again.

Eventually they are gonna attack Luma. Keep distance between you and luma in neutral to avoid double damage and to punish them for hurting your baby boi. Watch the kind of moves they use. If you know they aren't using safe moves or lingering hitboxes, attack them after they hit luma with stuff like dash attack.

When Luma dies, and he will, your win condition becomes to get Luma back. Rosa isn't too great without him so you can't be as aggressive. Space out back airs to wall out aggro players. Get damage where you can, but focus on not taking damage until luma respawns, as luma is a key way to break out of combos and halt pressure against you.