I was about to make a thread of this match up XD lol good thing I found this one
So basically what you want to do is keep stage control wich in this match up can be hard, both of these character can punish each other pretty well so your neutral game is key in this match up. Laser Marth when your far or at a distance you know he cant safely approach you, laser too close and you can get grabed or he could jump OOS and fair. Never make your appraches predictable, you always want to mix it up, used space aerials, laser approches, shine grab, nair for shield pressure (safer) and dair when you feel you can shiel poke him
When Shield pressuring Marth remember he has an UpB and it kills, also he has a few other options such as dair OOS
When edgeguarding Marth alwasy remember your bairs coming from bellow beat his aerials, also hanging to leadge I feel is good against Marth, what do is that I hang on and dependng what they do I punish, if the fair me while I have invincibility I let go and do a dair while going back to stage (feels awesome when you get them with this), but you bair is still safer cause if he is safe and spaces his aerial rigth dair wont hit him. Another thing that works grear against marth is Crouch Cancel, literally just stand near te edge crouch and when he does up be dsmash or dtilt, depending on the % both can kill so pick your poison (though I should say that if he sweetspots CC wont work XD)
You want to combo Marth with shine to dair to shine but only to low % atlest up to 30, higher than that dair to uptilt works great, when ever you want to combo him with dair remember to hit him with the soft hitbox cause the hard one will sent him downwards to fast to follow up the combo, I hear that at 0% upthrow and fair combos really well but I have never trd this x.x
(gona keep posting later on, taking small break)