He weighs exactly the same as Doc, so there's nothing you have to get used to (unless you meant his floatiness instead of his weight).
•
Learn to mash out the tornado (and read this thread:
http://smashboards.com/threads/rising-cyclone-ptp-frame-data-guide.384946/). Luigi's recovery is much less terrible with a good tornado. Learn everything about how the tornado works.
• Luigi generally has a lot of trouble approaching in neutral; he has to spend a lot of time waiting for openings. This mens you have to be confident in capitalising on those openings. Know your guaranted punishes and have good control over your movement.
• Be very careful with fullhopping; Luigi is far too vulnerable in the air. Practice quickly getting to the ground after combos and such.
• When using up-B to sweetspot the ledge, always try to tech; it's an option select so there's no reason not to do so. Be sure to get into tho habit of releasing up on the control stick after each up-B; Luigi does not ever want to do the jumping tech that occurs if you're holding up since you'll need to missile back to stage, which practically guarantees your death unless you misfire.
• Wavedash grab, down smash, or forard tilt are common approaches are a good approach. Forard tilt is very crouch-cancellable and shieldgrabbable, so be wary of that at lower damages or when the opponent is grounded. Tornado clanks with pretty much everything and lets Luigi act quickly afterwards, so that can work sometimes too.
• Against space animals you have an up throw chaingrab into up smash, which you can end early if you need to.
• I found ledgedashing something important to work on early; Luigi's is extremely good because it has lots of intengibility (it's possible to do a perfect ledgedash to a buffered roll without leaving Luigi vulnerable) and reaches extremely far into the stage.Has has pretty strong options he can use during it too (namely down smash and grab).
• Weak nair combos into a lot of things. Up aerial combos into a lot of things but not on DI away usually.
• Learn double aerials and short hop waveland aerials.Mix up your aerial choices and timings.
• If you have all the basic tech down, you might want to work on platform drop canceled nair. It's the fastest way to put out Luigi's best move; that's definitely a good thing.
These threads will be useful if you haven't looked at them already:
http://smashboards.com/threads/luigi-hitboxes-and-frame-data.306546/
http://smashboards.com/threads/luigi-trials-video.347163/
Read lots of Luigi threads, watch lots of Luigi videos.