The Marth MU is my best MU, 50% of Norwegians play him.
You want to make him commit to a move, and be ready to punish it.
Marths moves are kinda laggy.
Ganon can easily hit Marth with an U-air if he tries to approach with F-air.
If the Marth is a scruby Marth that just spams F-smash you should just dash in and out of his space to make him commit to a f-smash, then quickly punish with a f-air, b-air or grab.
I find that side-b is really useful against when the Marth tries to approach with n-air, since Ganon ducks under the hitboxes and is able to hit the side-b. side-b leads to a lot of such as u-air.
When edgeguarding Marth, the best options IMO is to either just hold ledge, or stand with your back towards him so you can tipman side-b recoveries that are used to high and close to the stage.
B-air works great if the Marth ends up over the ledge with up-b or you can use the Marthkiller if he tries to sweetspot.
You can get one regrab on Marth with d-throw if he is at 0% (not sure if this is guaranteed).
You can also get d-throw to Up-smash on him at around the 60% mark if he doesn't DI behind you. If he does you can B-air or U-air him instead.
I prefer fighting Marth on BF and DL64.
Yoshi's and FoD are ok, but he can get some easy tippers on those stages.
FD and PS are the worst stages in this MU IMO, avoid at all cost.
Hope this helps
