Against Mario, you have range, Marth's range over Mario's which gives Marth a major advantage in this match-up, and the ability of how easy it is to edgeguard Mario. Mario's Fireballs aren't the...best for a projectile, you can easily Jab them, f-air, or perfect shield them, and go for a punish. And approach with aerials, lots of them, they give Mario a hard time to counter, since Marth has an easier time keeping Mario out of his range, then Mario trying to keep close. Umm...really, if you learn how to space with Marth you basically got this match-up.