The game has been awesome so far I've unlocked a good amount of Characters.
Spoiler if you don't want to know some of the characters.
Snake-Easy just play a bunch of quick matches on the Shadow Moses Island Stage, takes 15
The hardest ones to unlock on there are Ganondorf(Beat Hard with Link or Zelda) and Lucario beat the target test, all 5 levels, with one character. I need to unlock Toon Link next.