Luma is what killed any motivation I may have had to play this character. Watching Luma die over and over in those videos like it was the most effortless thing made me realize how much of a pita it's going to be to keep the character in her non-gimped state. The very fact the she can be weakened period is a major turn off for me.
There are points where because Luma floats in front of Rosa, unless you roll, it's going to get hit, go helpless and likely die. You can't use your shield since that only protects Rosa. A neutral combo or a dash attack is all it takes. Rolling and running away doesn't get you KO's. Stalling alone is not enough to win a game. You have to put Luma at risk in order to do that.
This is even worse when it's alone. You'll be spamming B so often due to how defenseless sending it out makes it. Side special has laughable range for a projectile. It can't dodge or protect itself in any real way when it's detached. If only it could mimic Rosa's defensive abilities as well as her offensive ones. That would go a long way toward making it manageable.
Here's what happens when Luma gets hit with an attack with moderate/strong knockback. As you can see. It aint pretty. So basically the best advice for keeping Luma alive is just to make it sure it never gets hit ever.
Keep Luma attached. It's Vulnerable. Send it out. It's even more so. High level Rosa play will boil down to how well said player is able to consistently keep it alive so as to avoid the so called "Lost Luma Lock" because she is one of the only characters besides Robin that can experience a temporary decrease in damage, coverage, range and attacking options during a match.
THAT is her main weakness. The fact that the advantage cooperating with Luma can potentially provide can be just as easily removed resulting in not only a loss in the combination options but all the aforementioned losses as well . Dealing with that will be a Rosa player's main challenge in every match. When playing most characters you only have to worry about one thing. Not the case with her.