I think marth has a hell of a time against ICs actually. Your usual strategy of juggling is hard to set up for and they have a great punishment game in general. The "they just do" might even have some merit as a stranded Nana can keep you from juggling Popo in some cases. Although maybe I just need to learn the match better? In general I think any character that can nullify marth's grab game or aerial range game give him trouble, and since I don't know what I'm talking about past extreme generalizations and opinions for this one, I'd rather not give bad advice.
As for Luigi, yes he can WD at you quickly. But even with that speed, he can really only get any punishments that you give him. Luigi is like almost all low tiers in that he's weak to heavy shield use, and if you're really not sure what to do, light shielding is a decent answer. Even if he grabs you, there are respective DIs to make his damage potential off of a throw minimal. More importantly, nothing he can do can really put you into a terrible situation at lower damages, but you can do that back to him quite easily. If you focus on your macro game, you should be able to make luigi look very useless. If you focus on a micro game, you're giving luigi a chance to outplay you when you really don't have to. Stick to basic but effective strategies that are generally good vs most characters and show the opponent why your character is tournament viable and theirs isn't. stay away from character-specific engagements until you have a lot of experience at that match specifically.