To fight Rosa you pretty much have to get rid of luma first,otherwise you're going to get knocked out of your strings pretty often. For me the best way to do that is cross slash and back air, spaced to the point where if rosa is shielding its barely hitting her and mostly getting luma. If she does something unsafe and you're able to punish with dash attack, go for it, that'll send luma off the edge depending on where you are on the stage, and if not its pretty good damage. Other than that, just play super patient. Charge, shield, roll/jump away, just be aware of what she wants to do. You're not really in danger of your shield breaking so long as you don't sit and take lumas jabs endlessly.
Once luma's gone, you've got 13 seconds to do as much damage as possible. If she's shielding a lot, hit her with back airs till its low enough that she risks it breaking. If you manage to pop her up, bait out the dair/airdodge and punish accordingly. If she ever decides to up b, offstage LCS if its available. If not you can try going for the fair spike or just hit her with nair/dair, she has no hurtbox/invincibility on that move so your goal is to make her regret needing it. Her aerials have good spacing, so try not to get in too close.
Also yeah, against rosa FD is probably the worst choice.