Honestly I don't mind this matchup, as long as it's not on YI (or BF to an extent).
The most important thing to do in this matchup is to keep your playstyle fresh. This matchup only becomes horrible for Ganon when he lets Marth play a reactionary game. So if you're being predictable, Marth doesn't have to react to your actions--he just has to react to his predictions of what your action will be. Because if he predicts you correctly, he already knows what you're going to do as well as the best way to punish it. So all he has to do is space it beforehand and time it correctly.
On the other hand, if you're being unpredictable and perform an action that he doesn't expect, not only does he have to register your action--he also has to think of the best way to punish it, and attempt to space himself so that he can time it properly and punish it. It's much harder, especially against an opponent as intimidating as Ganondorf.
I know you're probably thinking that you should try to be unpredictable in every matchup, so saying it here is pointless. What I'm saying, however, is that it's a lot more important in this matchup than it is in most matchups. Due to the nature of Marth's spacing, Ganon has a lot of tools to punish him for even slightly bad spacing--but you need to be unpredictable for them to be effective.
One of these tools is Wizkick. IMO, Wizkick is more important in this matchup than in any other matchup, by far. It just punishes Marth's whiffed Fairs so nicely. Most of my Wizkicks will hit Marth during his vulnerability frames when he hits the ground, or maybe a bit earlier. Something about the way that most Marths space just makes this work so well, as long as you're not being predictable.
Also, iDA is really good against a Marth who's spacing with SH fairs. It's a quicker punish than Wizkick obviously, and it's safer. Staling a kill move kinda sucks though.
If you're diagonally below Marth (after knocking him in the air), baiting an air dodge can be pretty easy since Marth doesn't have good options when his opponent is below. Landing a sweetspot Fair after a baited air dodge feels SO good, and it will **** Marth's **** up. Especially if he already used his DJ.
Now to my favorite part of this matchup... edgeguarding. Marth is probably the easiest character to edgeguard, in relation to his spot on the tier list. Which means you need to get Marth offstage as MUCH as possible. Note that Wizkick does this nicely, so if you're trying to be unpredictable with it, try using it in situations where it will probably knock him offstage. Once Marth is offstage, Ganon's usual edgeguarding tactics work very well in this matchup (uair/reverse uair, drop off aerials, etc). He can side B stall to try to throw you off, but it really isn't that great. If you're going to be going below the stage to edgeguard, however, make sure you're ready to tech his up B. If you're not good at teching, then you need to practice teching. Of course, don't TRY to get hit by the up B just so you can tech it; just be aware that it will happen somewhat often when you're fighting Marth offstage.
But one thing that I find effective (but kinda risky) is spiking Marth when he recovers with up B. More specifically, when you're on the stage, full hop off the stage, then fast fall a Dair directly into Marth's up B hitbox. Marth's up B is only out 1 frame I think? And Ganon's stomp is out for 6 frames. If Marth comes into contact with Ganon during those frames, he dies. And the best part is, you'll never get stage spiked when you space it properly. If you hit, then you'll get hit by a very weak hitbox which, if anything, just helps you recover. If you miss Marth with the spike, then you'll likely get hit by a stronger hitbox, but you'll probably be popped straight up and not into the stage. If you miss Marth altogether, then you still have a double jump and an up B. Luckily Marth just grabbed the ledge, so he probably won't be able to regrab it fast enough to edgehog you.
As far as the rest of the matchup goes... well I'm pretty sure you guys know the stupid stuff Marth can do to Ganon, especially if they play patiently. One thing to note is how hard it is for Ganon to get back onstage on YI or BF when he's hanging from the ledge. Marth covers all of his options very nicely on these stages. So make sure you avoid those stages... if I have to choose (I usually do), I'd ban YI and let him take me to BF, though that's likely because I dislike YI in general.
All in all, if you're good at gaining momentum in a match, you'll probably do well in this matchup. Just make sure you don't fall into a rut when you're fighting Marth, because he will systematically destroy you.
30:70