Here's a great way to unlock all of the characters in the game rather effortlessly. It should only take around two hours. Since all of the hidden characters can be unlocked in some way via Brawling (versus mode), you can slant the mode in your favor to do rack up Brawls quickly and effortlessly.

Start by selecting the Special Match mode. Set it to Stamina and Flower modes, and on the character selection screen give the CPU 1 health and your character anything over that amount. (Make sure it's on 1 Life Stock mode). Set the map, at least until you unlock Snake, to Shadow Moses Island. Play with these rules for about two hours and you'll rack up 500 matches in Brawl, more than enough needed to unlock all of the characters. Just remember -- you have to beat the character you're trying to unlock!