Careful of holding shield, because Shield Breaker got a buff. Go for spot dodges and rolls instead. Not that theres anything wrong with shields, just keep them short and sweet.
Try to stay away from PK Fire unless youre punishing a landing. Marths love their aerials, and as such they can have a bit of endlag. Use that to your advantage. That being said, make sure you cancel your endlag or land in a safe spot, as a Marth can also take advantage of your endlag as well.
Marths are also absolutely vulnerable to PK Thunder, as their recoveries are linear and predictable.
Marths aren't very heavy, and backthrow can kill at around 110%-ish from near the ledge; But theyre heavy enough to be strung along with your own aerials. Just be careful because a miss could lead to a punish. (I think he's a little bit lighter than Inkling and Villager for reference)
Make sure to keep yourself very well spaced. Practice with a CPU if you can't work out the hitboxes. Theyre very big and very fast.
Just like with any matchup, keep your cool. Sometimes, getting tilted is exactly what your opponent wants you to do. Since Marth has a good punish game, getting mad and using your stronger attacks (that have a bunch of lag, like the bat or PK Fire) will only get you thrashed.
I hope this helps.