Seeing as how there's already a MU/Q&A thread, I'm going to close this to reduce clutter. You are more than welcome to get more help there.
To answer your question, water shurikens are actually pretty good at doing chip damage to Luma and when fully charged, will go through Luma and hit Rosalina. I've also noticed that SHFF nairs can sometimes dislodge Luma and send him flying a little back. Hydro pumping her predictable recovery path works well too as it'll send her high up and given her relatively slow falling speed, it's enough time for a punish. With Luma present, uthrow-uair is generally harder to do since Luma can input attacks while Rosalina is in hitstun from the uthrow. Get used to powershielding from grabs,