I really wish I had more to add to this matchup than I do. Mostly it's because I don't do this matchup outside of friendlies. I even have the best marth in the region in this state and we're buds. But for real, it's such a bad idea to do this **** that I play my crappy donkey kong instead.
Things I do know.
STAY AWAY FROM THE LEDGE. On paper (theorybrawl that is), grabbing the ledge, with marth at the ready, is a stock. There's no reason, as far as movesets are concerned, that a good marth should EVER let you back on to the stage from this position. Now we all know that this isn't the case in practice, but that's just because everyone is human and can make mistakes and be influenced by faints and switches. If you find yourself in this position you need to be completely unpredictable. Take notice of his position and figure out what he expects of you, and do the thing that makes absolutely the least sense. If you do something completely ridiculous you'll still get hit half the time, but thats an improvement.
In a matchup against a campy, competent marth that knows the matchup well enough it's not uncommon for you to take like half your damage from the ledge.
The way to recover in this matchup is as follows. Do your normal momentum cancel routine, generally fair to missile, then immediately double jump and cyclone. Get as high as possible, because it gives you more options and allows you to be less predictable. Then, spam tapped missiles. Don't charge them. The taps give you a chance for a misfire, and misfires are unpredictable, therefore they throw people off. Throwing people off is what you want the very most since luigi is balls slow and easy to juggle in the air.
If you get a misfire and are still very high, but over the stage, feel free to missle too far, or back or any way you want. Your goal is to be weird and confusing. Once you come back onto the screen your goal is to TOUCH THE STAGE at all costs. You absolutely do not want to be hit to one side or the other before you hit the ground. This would put you in a TERRIBLE pickle, since you'd be off to one side, probably pretty low, with no double jump and NO RISING CYCLONE of any sort. In other words, risk even a SURE hit, maybe even a free KO, just to touch the ground. Because maybe they'll **** up and miss or choose the wrong move. If you get faired out of your recovery at this point you just die.
This, for instance, is the proper (IMO) way to recovery against any character that can shut down your low recovery. Marth, mk, kirby (KIRBY!), clever gws, specific warios, sheik, or a fox who REALLY knows what he's doing. Kirby ***** tornado hard, just about as hard as mk. And the only fox I know that's figured it out is OS', but fox easily has the potential to absolutely force a high recovery. The best fox I know of, zeton, doesn't know about it. But then again he doesn't know the matchup.
Now, the reason I don't like this matchup is because you have to play so much better, work so much harder, and be so much more PRECISE than the marth in question to beat him. And precision is a tough nut to crack, since marth's whole game revolves around that concept. You MUST predict the fairs and powershield them. After that you may enter brawl. Paper rock scissors is the name of this game, and in this matchup you're only allowed to play after you've powershielded one of their aerials. From that point you have a few options that work, a few aerials or moves that beat his aerials for speed, or the option to guess a no-aerial, and attack his landing. Also if you correctly predict a nair, you can just tornado, and it eats everything else. If you're poifectly spaced, and he throws out an empty fair, you can tornado under, either to try and hit him or (better idea) just get to the other side.
The real problem with this game is that after he finds out how to get past your fireballs (and trust me, if you're not getting *****, he'll have time) every aerial you DONT powershield either hits you or pushes you closer to the ledge. You really can't get by him if he's doing his **** right, and you just get closer and closer to massive damage. He crowds you to the small part of the stage wehre you have no reliable options to leave except the edge. Have fun with that one.
Overall at least 70/30 marth. Since all numbers are subjective, I look at 7030 as "a matchup you're better off trying with an inferior secondary with a better matchup". Bad enough that your expertice with your main won't help enough to get the edge back.
I have two tips, things that work.
JUGGLE. Marth is one of the easier characters to juggle, he doesn't fall that fast and his airdodge is bad. On top of that his dair is ****ty and small and slow. Because of this marths are reluctant to use dair and most of the time will airdodge. If you catch one in the air, jump up and growl, they'll airdoge and you get a hit. Just be far enough below them to not get airdodged through.
At all costs, bait the dolphin slash. The first time they dolphin slash you out of a jab cancel, start jab to shielding. Do it half the time if you have to. Get that **** out. If you get lucky and shield one, go for ****ing broke. This is your shot, do not waste it. Keep in mind, they use the same mindgames you do for missed upbs. Don't for a second expect them to land where you want them to. If they have an option to land on a platform or on the stage, hit them out of the air. Also keep in mind that your shoryuken works in the air, just overlap and input, **** does like 26 damage and kills marth at 85ish.
Last tip, if you find yourself forced to recover low (or you just forgot) and the marth actually leaves the stage, waiiiiiiiit. Wait some more, wait until its just about too late. Either they'll chicken out and jump back up or they'll do an aerial out of guessing. If they do the dair, just cyclone up and kill them. It's nearly certain to gimp, just remember to stay on the edge. If you're below 80ish, you can recover from a DS stage spike. It's worth the risk, unless you know that **** really well, then just roll when appropriate.
FOREALS last tip. Play donkey kong. It's way better. Against marths that know the matchup, they hate it. Against marths that don't know the matchup, it blows their mind that someone can consistantly outrange them, then they panic and lose badly.
Holy cow this turned into a wall of text. Well, dtp, if you want any of this for your summary, feel free to pick/choose/chop how it feels right for you. I trust you. If I were you though, I'd ignore it, since I'm a scrub.