This matchup is pretty tough, cuz it's easy to be gimped by Marth. If the Marth is spamming SH double Fair, shield the first fair and Uair OoS. Another tactic with good Marths is to space Nair against Ganon. M2K finished me with this one cuz I hadn't seen it before, also against Chaddd. I met another Marth recently who started Nairing right through me and landing behind my shield straight into a reverse Utilt. It seemed like an unbreakable tactic (All I could do was roll away) until I started crouch canceling instead of shielding (DUH!)
Don't let Marth get you into the air, because then he will have all the control.
Don't forget to light shield edgehog.
Also, I recently went to a tournament. I've mostly got only exp in friendlies, so it was a big learning experience. I got nervous and bombed my first set, but then I remembered that I'm the best, haha and that confidence got me back in the game and into 3rd place. Check out the vids here:
http://www.smashboards.com/showthread.php?t=237334