A good Marth will wall your approaches out with his better range, but he can't do anything to you outside of that. Zone the crap out of Marth with grenades and mines; this also limits his greater movement to an extent. If he gets caught in an explosion, then you can approach relatively free. Tilted DACUS could be an option as well, but the Marth matchup is less about approaching and more about pressuring him to approach in an unsafe fashion.
You can try out gimmicks like baiting him with a charged mine (or grenade) to shield: get your timing down and you'll shield the explosion and Marth will eat it and get popped up, which is where you want him anyway. Plant another mine to limit his descent some and use Uair, tilted (for wider peaks) mortars, and wavedash grenades to keep him above you.
Grabs are great, as you can upthrow chain until around 30%—at which point you can stick, ideally with a wavebounce to avoid the punish; after that, downthrow tech chases work to rack up damage: you can tranq him during this if you missed your first stick attempt, grab -> pummel (if <10% under kill percent, otherwise the quick throw is good to throw off DI) -> upthrow -> detonate. Should kill on most stages around 85%, but I'd have to test that number as right now it's based on the data I've collected on the Mario (his upward percents tend to be similar) who dies to the combo at 75% no DI on FD.
If you start getting juggled, C4 (to uair or dair/fair spike if the situation arises) to cover your descent is a good option (you can Cypher to get sufficient space for the 30 frame execution); if Marth is at high percent or stickied, you can bait him up and probably kill off the top since Snake lives a bit longer in that way.
Never ever recover in his f-smash range, because they will never miss a chance to use it. You can go just outside it and fair/dair/bair above him, but your best bet is usually to sweetspot since the Cypher tends to block (stage)spike attempts, and it'll conveniently stun them if they're positioned incorrectly (and by that I mean naturally, near the edge) which combos into the <100% getup attack.