I mean... I'm fairly new to Rosalina. But one thing though, Rosalina isn't that MLG O.P doritos characther if you remove one aspect of her which is Luma BUT DO NOT GET TO THIRSTY, YOU CAN AND WILL GET PUNISHED.
But yeah try to kill Luma then move in to murder Rosalina as she isn't all that great by herslef. However don't try to attack Luma when shes with Rosalina as you will get punished. Though when Luma is by himself don't be afraid to smash him to another dimenson. So yeah... only attack when Luma is gone