What professor AirFair said is very true, but I would also work on aggressive dashdancing and sheildstop aerials. It might just be the Falcon main in me saying this, but these two things are two of my major tools when fighting Ganon with Marth. Admittedly, I don't have any accomplished, veteran Ganons in my area, but I'm pretty sure Marth shoots slugs that kill every pig it hits, no matter how big. What I mean by this is that Marth has tools that, if used accurately enough, can beat any Ganon simply due to disjoints and combo opportunities. Let me break this down.
What I mean by "aggressive dashdancing" is not necessarily an offensive option, but in fact a veiled defensive one. If you watch high level Marths like Mew2King and especially PPMD, their Marth's dashdances are terrifying, even to the spectators like me watching from home. The sheer suspense and ambiguity of their movement, reinforced by their devastating, soul-crushingly good punish game is enough to make the opposition break down like a witness in an Ace Attorney game. Since Marth can outrange Ganon with good spacing and smart movement (and the Ganon has to know this), "aggressive" dashdancing that is threatening and scary is going to force the Ganon to lash out and do something out of both the monotony of your dash dance and, at the same time, the threat they're feeling from it. Use Marth's tools to psychologically wear down on your opponent's patience and force his hand through false threats with a teeth-chatteringly scary DD.
Sheildstop aerials are an outstanding mixup in this MU, and one that few marths I see other than PPU using well. The shield stop can be utilized by itself or in junction with your awesome dashdance that's making the player next to you question why he ever picked up a GameCube controller. Suddenly breaking your dashdance with a sudden dash towards -> shield can cause some people to freak out and instantly fair your shield or something, or they'll just run away. In any case, you've used Marth's superior options to gain an advantageous position against a character that relies almost entirely on the opponent messing up to win. If they have a mini spaz attack and do something like Fair or run up and try to grab you, your sheildstop has just turned into a sheildstop aerial. Fair, Dair, even retreating Nair OOS can all work here, depending on what's running at you. Pick your favorite. If they DD away because they're absolutely scared ****less by Marth and his huge sword, congratulations, you've just established better stage position. Beware for the inevitable Wizard's Foot to follow or some kind of waveland in -> Ganon stuff.
WD OOS is also an essential tech that is useful for every mu, not just Ganon. Sheildstop -> WD OOS is probably the followup you want if they ran away from you.
Hope this helped, everyone else has given great advice so far so make sure to study that a bunch too. If you have any questions, hit me up again. Good luck!
