Apologize for the quality:
I've known about this for 6 months or so and I've been sitting on it, but I decided to go ahead and share. Luma's forward air sends opponents pretty close to straight up and Rosalina can double jump fast enough to reach them before their hitstun ends if she autocancels the Luma fair. This leads to guaranteed kills off the top around 80%-100% depending on character, rage, and move staling. DI can be followed fairly easily, and it's usually possible to hit with both Rosalina and Luma's uair, resulting in a 3 hit combo.
The trick is actually hitting with the forward air. It's a pretty small hitbox but you have a lot of options. I'll just throw out a couple approaches I use.
SH rising dair to autocancel Luma, covers rolls, if the dair doesn't hit you can hit them with the fair and chase, as sh rising dair autocancels every luma aerial besides uair.
SH delayed air dodge into fair. You cancel your air dodge landing lag with a Luma aerial. The latest you can air dodge and still do an autocanceled luma aerial is just a tiny bit before the peak of your SH.
I don't have video to demonstrate it but this last one is an important technique that probably should get it's own thread, but since history has shown I have a tendency to just lump a whole bunch of things into one thread and write the longest sentences possible, here we go. Auto dash luma autocancels. Simply jump, hold the direction you want to be running when you land, and flick the cstick for the desired aerial for luma right before landing. This means you can RUN with a back air or forward towards your opponent automatically, without having to time it out. It's extremely good.
Anyways I'd love to hear your ideas on ways to approach with Luma's autocanceled forward air, that's partially why I'm here posting it. I've hit a bit of a dead end! It's a kill combo off a lagless aerial, which sounds amazing on paper, but I can't always land the forward air.