This is the way I did it.
Make sure you have enough Hyper M. Guards, L.Shroom Shakes (or Final Elixirs), and Kerokerokolas left. You can also throw some Found Illusions in there to help out with the timings, but keep in mind that they have a lot of turns so a single Found Illusion will not be enough so try to save as many as possible.
For most important equipments, have Mario use the Ultima Weapon, Peach could use Lazy Shell and a Speed Bracer (only if you don't use both Mallow and Peach in the same party). The rest is up to you but make sure you equip an armor and accessory combination so that you half elemental damage, and you're immune to mortal blows and status effects.
Now for the strategy, each Luma have 60K HP. Try to deal enough damage to each one of them so that you can defeat them all with a single turn. That way, you'll have more turns to attack her.
Here's were blocking is really important because you don't wanna waste turns reviving people. She have this galaxy attack where she deals massive damage to a single party member. However, if you block that, is more that guaranteed that everyone will survive her AoE attacks. Just heal with Peach and make Mario your primary damage output. He deals INSANELY amounts of damage. Also, make sure you are buffed at all times and with Auto-Life. That's why you should use some Hyper M. Guards.
Don't bother using attacks that lower her defense because she seems to be immune to that. Just keep on repeating this and you should be fine. Like I said, this fight relies A LOT on blocking attacks so make sure you're good at that.