You have to be good at 2 things to be a dominant Luigi player in Brawl...
1. Able to powershield EVERYTHING
2. Intelligence and lots of Mindgames
Its good to have those things to be dominant with any character lol...
It's impossible to have reflexes so good that you can power shield every hit. Power shielding a hit is mostly luck. Nobody purposely waits for the last second to shield, they shield as soon as they can. Spotdodging is a lot more reliable. But you also need to know when to hold down the shield for attacks that hit multiple times.
Granted you might not be able to powershield EVERY attack. But if you predict moves its not too hard to do.
I suggest you read the guide on luigi first, that way you can ask more specific questions that other luigi mainers will have an easier time answering questions.
Hippiedude92 has a thread devoted to specific questions and he knows his stuff about luigi.
Make sure you have basics down like shorthopping into various moves, RARing, and doing two aerials with each shorthop (luigi is floaty enough for that).
Just give him a try with some lower level computers and practice putting yourself in different situations. Watch some luigi videos and try to mimic their style while adding your own flair into it.
Again, this will make more sense to you after some practice