Blogiversity.org

1. Finish Classic Mode on any difficulty

2. Get Marth in the Subspace Emissary

3. Complete 10 Vs Matches

Marth is a very easy character to unlock. If you want to unlock him really quickly, just set the stock to 1 life and battle a friend in 10 vs matches. You can also unlock him by just beating classic mode on any difficulty. I'm not sure yet how far into Subspace Emissary you have to get to reach him, but you will encounter him on the way. If you have a long range character when he challenges you, just stay back and blast him with projectiles. He has no long range capabilities to speak of, so just keep your distance and you will be fine. After racking up some serious damage, you should be able to finish him off and unlock him.

I just started playing some Brawls with my brother and we unlocked him really quickly. When you play with a group of people, the winner of the match will have to battle Marth. We downed him quite easily. You just have to exploit his lack of ranged attacks.