When Marth approaches, I like to crouch, this pressures him into fast falling the majority of his air-game, which limits some of his aerial mix-ups. If he enters your bubble vertically, stuff his attack with your upb (may need to angle), if he doubles his aerials, shield the initial blow, and follow up with a shield grab, jab, dtilt, dasha, or upb, crouching also usually guarantee's a whiff.
You need to capitalize on your juggle traps, and edgeguarding. And you shouldn't really be approaching, just punishing spacing mistakes. Every time he advances, simply walk just barely out of his blades tip, and punish the cooldown.