Well I guess if we're doing Luigi...
Luigi's aerials are strong and they're fast. Very dangerous. Watch for his shorthop double aerials - it'll usually be something like a fair or a dair into a nair. If they use an early aerial that's not the nair, they can waveland out of it, so be on your toes. Don't try to grab unless you're both in range and you know that he won't hit you with an aerial. If he grabs you, chances are he'll dthrow, from which he'll most likely try to follow up with a fair or a dair, both of which hurt, so DI as best you can away from them. Behind him should help you avoid the fair, but if he wavedashes into a dair, he'll probably be able to catch you, so watch out.
Luigi's main approach will most likely be the wavedash dsmash, ftilt or grab. Sometimes he'll use the... Luigi Tornado? I have no idea what it's called. In any case, his wavedash gives him very fast movement speed, but only on the ground. In the air, he's slow. You can capitalize on this by throwing Shadow Balls to impinge on his movement, but watch for waveshielding. Spaced dtilts should also be able to choke his movement. If you can bait him into going into the air, he's committed for at least thirty-one frames - that's a shorthop fastfall. Once you manage to get him into the air, try to keep him there. His dair hurts, is fast and has a lot of priority, so be careful. Your usmash should be able to beat him out there. Take your time, he's not going to be faking you out with a doublejump since he's so floaty. Watch for the hover-then-drop Tornado though.
Once you get Luigi off the stage, you should try to be aggressive and go for the kill. If you can spike him a good distance out, that's going to be the stock because his Meteor Cancel is almost completely worthless. Dsmash is an excellent edgeguard as he can only go straight up with his Super Jump Punch. An easier way to edgeguard him would be with the lightshield edgehog. The same one people do to Marth. Actually, this edgeguard works against every character with only a single hit on their recovery and possibly some others. If you do decide to go with this option, be prepared to jump out and bair him if he decides to be clever and airdodge to recover instead. Oh, and be careful of misfires. They suck.
If he manages to latch onto the edge, try baiting out a ledgehopped attack. A lot of Luigis love ledgehopped fairs, but his floatiness can really screw him over here. If you see him ledgehop an attack by dropping and immediately jumping and can avoid it with a dashdance or a short wavedash, you can fsmash him back out and suddenly he's minus his jump. He can ledgehop double aerial too, so be careful of that if you're going for a shieldgrab. In this situation, if you have a charged Shadow Ball, you can shorthop back and throw it at him, which should be easier than the fsmash. Also, charged Shadow Balls should work well against his Green Missile recovery because of its linear nature.
That's about all I can come up with at the moment...