Umm... well, sorry for the bad news, but fighting Marth is hard
Fortunately, there are methods and tricks.
First off, his d-tilt is really good, but you have a simple answer to it, which is SHFFL'ed F-air. It requires moderately good spacing to achieve, but you can hop over the tip with your f-air. Odds are he'll CC and you can have a free d-smash, jab grab, or tech-chase.
It's also possible to wavedash in from a distance and grab him to punish when he misses, but you need to predict both the spacing and timing of the tilt, as well as execute a long wavedash. I don't think it's worth it to try this, really.
His f-airs are the devil, but again, spacing and timing is how you can beat him.
First off, the f-air is punishable as long as it barely misses you. You can slip in with dash attacks and grabs, or if you're really sneaky, wave-smashes. Another trick is to use long wavedashes into shield to try and make him land a deep hit on your shield. This reduces hit stun and almost guarantees a shield-grab.
His f-smash is really good. The answer is do your best to shield it, then wavedash from your shield to punish. Any Marth that uses the f-smash too much will lose to you purely from grabs.
His grab game is strong, but defeatable. The hard part about it is that he has massive range on the grab and your shield is very important to you as an IC player... but if you keep a couple tips in mind, you can probably punish his grabs pretty well.
1) If he grabs Nana, smash him or grab him during lag. You can shield grab his throws if you're in close enough.
2) If he grabs YOU... keep shielding with Nana and have HER shield grab instead. If he throws you anywhere but up, you should recover soon enough to combo off her grab.
His forward+b is really good. Fortunately, the first hit or two are susceptible to crouch canceling. You can get CC'ed grabs if he only decides to do the first hit, or CC into your shield and a shield grab if he keeps going. Remember that the different forward+b's make him move around a lot, so you need to consider where his body goes when he does the different attacks. Sometimes you can grab and the ending animation will pull him or push him out of reach. And they hit in weird places on your shield too, so watch out for that as well.
I also strongly believe that blizzard is one of your best friends. Try and have Nana blizzard from rolls and dodges, have her Nanapult to a) land blizzard hits and b) act as bait that lets you land whatever opener fits the situation.
I don't really have a good method for edgeguarding him, but I'm toying with some weird ideas ATM. I'll post about them if they wind up panning out.
Does this help a little? It's very much a spacing battle and that's hard considering Marth's reach, but I believe it to be winnable.