So, I'm a Samus main. This is a particular matchup I've been studying a lot recently. I think it would be pretty helpful to have a dialog about the match-up.
As a Samus in this match-up I'm looking for a handful of things in order to take control of the match. First of all, Randy and misplaced fsmashes, wave dash out of sheild and downsmash puts marth off stage and gives me, or the samus, some widdle room to work some edge guarding or a straight out kill. If I've managed to survive for a long time I know Marths are antsy so there will be a lot of approaching sheilds.
Second thing I'm looking for is misplaced fairs, too deep on samus that can be cc down smash or dtilt. Dtilt is a move that at low percent can lead to platform tech chase dairy, which can string into nice bairs or nairs. Marth can counter this with well placed fairs and immidiately shielding. A downsmash on sheild is punishable by Marth pretty easy, so watch out for that. Keep in mind that the meta will evolve and aware samus' will start sheild grabbing.
The edge guard for samus is fairly easy. Try to avoid using the jump, but well placed missiles and charge shots can force that jump, allowing for possible Bair edge guards. Going low will result in invincible nairs which basically wrecks Marth.
As a samus I have a lot of trouble getting from ledge to stage against a smart marth who will hold their ground and move intellegently against most if not all of samus's options. Fairs are most common, simply wave dash back f smash most the time to beat her out until she smarten up. Although samus can't do much from ledge against marth regardless, roll is react able, ad is tournament winner. If your spaced too far back you can pull a Hugs and land gently on the edge and bait a swing. Dtilts also wreck samus on edge and while recovering. Don't rely entirely on the dtilt, after you hit one or two, grabbing ledge or dair will end things quickly. I would NOT advise going out off stage against a samus. As soon as you leave stage she can inturpt with an up b. Grapple and wall jump mix-ups also can get in your way.
I'd like to hear from a Marths point of view how the samus edgeguard works. I've recently picked up no impact landing, does this ever throw off any of your options?
In neutral, marth dominates if he can play around the cc. F tilt and jab on grab is mostly punishable by marth who can simply grab ad far as I can tell. The juggling is also silly, but a lot of samus mains are used to when to bomb and when to fast fall nair to throw off the marth. Basically try to be as uncommitted as possible when juggling and read the samus response.
A smart samus will di out most Marths throws and hits, Ken combos are rarer on those players, doesn't seem relyable. To me.