First tip I can give is practice spacing your nair. Dtilt after to punish shield grabs, or whatever. You gotta be flexible with the nair. You can approach with it, you can move backwards with it, or you can stay still in the air. Know where your opponent is moving and aim the end of the attack there so you have enough room to not get punished after.
Second, practice your recovery a bunch if you mess it up. Try as many weird angles as you can think of, learn the different ways up B works, it can go straight up, up and over, or nearly horizontal, and you can B-Reverse it.
Third, Forward smash is pretty safe on shield if you have Luma and you space it well. Learn it's range, and just use it when your opponent doesn't have many options, like when he's backed into a corner. It kills early and it has good priority. It's still laggy, don't overuse it when someone's in your face, it wont work hardly ever. F-tilt or D-tilt to get people out of your face.
Finally, practice uair strings. Watch your opponent, learn their airdodge patterns, most people do the same thing over and over. If you whiff your uair and they airdodge into you, do a dair if they're in front/below you, but only if you have Luma. Uthrow into utilt into uair is a good early combo, later on you can do uthrow to uair. Uair is less effective when you don't have Luma, so stick to utilts and the occasional surprise uair. Use Luma in your uairs as a buffer between you and your enemy in case you misplace your uair, Luma will save your butt over and over. Fast fallers can also be combo'd from uthrow into jab attack, which may be better than utilt, depending on how you feel.