I have to agree with the above post. Very nice constructive help.
Some key things to play with is if you ever do roll, always use the c-stick to roll. This lets you will much faster.
When i play My Luigi, i rarely roll, but thats just because i spam wavedash >.<
When you do use your sheild, becareful for that grab, because it can come very quickly against good players.
Also, grabs are great. They get distance, they set up for some fun short-hop comboes and they are great because Luigi's Wave-grab can cover some insane distance.
Also, always try to use your Forward smash on Foxes, rather than you up or down smash. Fox is hard to kill upward.
Luigi's greatest strength is the amount of aerial atacks he can fit in with his 2 jumps. Abuse this feature. Juggle juggle juggle with your aerials, use your B-Air to edgehog, use your F-air to edgehog, and your U-air.
Try practicing using your wave-dash into a shorthop. This can be difficult but pays off when you pull off an insane juggle across the stage.
Also, tonadoes are awsome to use against quick characters. With good throwback and great cover area, your tornado can seriously piss off opponents.
And dont waste your time usin gmoves that never connect.
Fast falling as Luigi is also vital, as it can free up more chances to fit in some damage.
All in all your Luigi does need work, but dont finsih reading this post thinking you suck, because you dont.