Hmm...
1: Fundamentals. It goes without saying that you can't mess up wavedashes, period. You get punished and miss a lot of stuff because of flubbed WD's, so my first spot of advice is to practice those.
2: At lower percents, I would recommend down-throw up-smash. This puts Luigi high up which lets you get the opportunity for up-air. It's one of the few things he can't just out prioritize with n-air. The aiming is tricky; you have to be under him but SLIGHTLY behind. This way he can't b-air, d-air, or n-air you out. It's his weak spot in the air, pretty much. You have to watch out though, because Luigi is floaty and has crazy air control, so you might not get into that sweet zone.
3: General spacing. You wander into attacks a lot; this is very understandable against a Luigi because he's zipping everywhere and his speed makes it hard to tell where he's going to go, but it's still something that has to be improved. Pay a little more attention to your opponent's movement and where you are on the stage in relation to him.
It's all kind of basic, but it should help, I hope.