The other problem with Rosalina is that unlike Duck Hunt and other zoners, there isn't really any notable openings for the side B kick. Whereas you can kick through the clay disc or gunman on reaction, or through Link boomerangs, the only thing you can try this on is Luma shot, which isn't actually that laggy.
If you're using bananas against her, you need to have a banana in hand and use it when luma is separated from her. A lot of you have been saying not to leave bananas on the ground, but I find that only to be the case when not near her. If you have a Banana nearby, and are in close quarters, she CAN NOT risk a gravitational pull or you get a free punish on it, despite losing the banana.
It's a tough matchup, but we have to remember Diddy has solid normals. Most of my game has been spacing with Bair, baiting luma out and then going for the grab / Banana game if they send luma out with a luma shot AND I get in. Good Rosalina's will keep Luma with them at mid - close range though so it's tough.