Neutral B is good at hindering Marth's approaches.
If he spaced DB poorly you can DI it out and you'll avoid the 4th hit, and when it whiffs you can walk up and dsmash/dtilt.
PS-->utilt/sh uair. They are good.
Don't airdodge into him at the edge, he'll dtilt and stuff.
Pressure offstage with lasers and fair.
Take him to FD/PS1.
Don't charge dsmash from neutral position, he'll sh over and tipper nair/fair you EASILY.
If he ever jumps but you know he's going to land away from you, try to punish his landing with sideB.
Dash attack isn't safe on hit below ~20 (from ~25 onwards dash attack --> utilt true combos, from ~40 onwards, all tilts and dash attack true combo out of dash attack...so below 25 if you dash attack him, shield cause he might upB)
Dakpo likes usmash OoS in this MU, and he's good and not dumb so you should try it out and see how it works. I've personally never tried it so I have no input on it.
Your OoS options from DB3 on shield are shieldgrab, utilt, dtilt, JC usmash and sh uair.
That's all I've got.
Posts in the ZSS MU thread are probably full of bad info. That ****'s old. And if there's a Marth arguing in there it's probably Shaya and Shaya sucks, so...
Edit: Oh..full jab combo works but not really.
What I mean by that is, Marth is one of the few characters who can't simply hold the shield button and PS jab3. However, he CAN upB you after jab2 connects.
So yeah, pay attention to upB habits as a response to jab/dash attack. Jab1 cancel is +1 on hit so if you jab1 --> hold shield I'm fairly certain you'll PS if he tries to upB you...I think that's how jab cancelling works anyway.