Try to better your spacing. Ftilt from afar and dtilt from up close. Grab when dtilt may be shielded or when it's a sure thing. If you shielded marth's shorthopped fair, and he's still able to airjump or do another fair, I suggest you roll/spotdodge. Also counts for rising nairs. You can only shieldgrab falling aerials, not rising ones. And good spacing on Marth's part avoids that. So trying to shieldgrab is bad if it's not safe.
ftilt, dtilt, grabs. Aerial marth may get hit by fair/bair. Jumping into dair from the edge likes to work against most characters. Edgehog, learn to chaingrab effectively, and that's that.
I suggest you don't look for a counterpick because of Marth. It's better to think about bigger problems, like Diddy Kong. And don't get into counterpicking MK yet. People still haven't figured out who's good against him anyway, so blindly choosing a counterpick for him isn't a great idea.