Perfect pivot bairs are good for getting rid of luma and they are easy to hit luma with and hard to punish if spaced correctly
I don't Pivot perfectly, in fact, I prefer the Imperfect Pivot (since it gives a little more distance).
And why do you have to Pivot the B-air? A skid-cancel B-air should be fine.
As to some tips vs Rosalina:
While Luma is in play and with Rosalina, focus on Luma. That means play footsies with Luma more than Rosalina because Rosa will be putting Luma between you and her. Also, mind which attacks you are using and how much damage Luma has taken. A fresh Luma won't move very far when hit by weaker attacks, so either wear it down a bit with safer attacks or
make sure the attack you are using has some oomph to it (thanks Asdioh).
When you're off-stage or being juggled watch out for Rosalina's ridiculous U-air and D-air options. It's tough getting back when below because Rosalina's D-air can dunk you or flat out KO you and using Air Dodge will only make it harder to get back up to the stage. Final Cutter is easy to intercept with her D-air, so think twice when using it. Likewise Rosalina's U-air can flat out KO Kirby and has a large disjoint. Not only does it KO, but it can KO at ridiculously low percents, especially when Rosalina has Rage built up.
In the neutral game, play safe as possible. You can get more risky if you've taken care of Luma and it's just Rosalina, but it's still advisable to play safe and look for openings. You want Rosa to make the mistakes and you really need to get those optimal punishes when she gives them to you. Rosalina is very light, so you can take stocks from her with U-throw relatively soon. Try not to be too panicky because Rosalina likes to capitalize on that and get her U-smash or F-smash in that way.
Getting Rosalina off-stage is very good. Not just because of the usual benefits from getting the opponent off-stage, but because Rosalina doesn't have a hitbox on her Up Special when she has to use to to recover. This makes D-air so much easier to gimp with compared to all those guys who knock Kirby's D-air away like it was nothing. Additionally this makes Swallow viable to use off-stage to intercept her recovery (at least when you're a stock ahead). Be mindful of her mid-air Jump since she can use that in conjunction with her Aerial Attacks to mix up her recovery options and possibly knock Kirby away.
That's pretty much a quick run-down of the Rosalina & Luma match-up.