Don't try to punish a landed NAir. Roll away after it hits you. It's a guaranteed grab if you try to punish.
A2Z is very much right, NAir is Satan. Actually, it's worse than Satan. That means it wants to usurp us. Dancing Blade is a *******, and is very hard to punish, of course. Don't put yourself in bad situations.
Marth is definitely a MU where we lose on their CP. You MUST win game 1.
I'd ban FD, personally, and take Marth somewhere there is a lot of space. Stadium 1 can work, but Stadium 2, Norfair, or even Pirate Ship can work, though if you play the water camping game he has a fairly powerful spike he can also take advantage of. Regardless of how well you think you might do, Pictochat is a bad option.
I figure I might throw my two cents in about banned stages, since technically if they agree you can take them there, and the Marth is unlikely to know much about Ganon or banned stages, so this may provide an advantage. Rumble Falls would work very well so long as you can FoG from an airdodge. We have so much more vertical movement than Marth with it. It's also very large, so keeping away should work too. New Pork City is generally a bad idea. Marth is faster than we are, falls faster too, so he can circle camp us all day. Don't let him convince you to go to Skyworld or Hanenbow, nothing silly like that. Familiarize yourself with RF and ask every Marth you ever face if they wouldn't mind playing the match there. The worst they can do is say no.