In Marvel vs. Capcom 3: Fate of Two Worlds for the PS3 and Xbox 360, you earn PP (player points) in arcade mode, training mode, and during online multiplayer battles. Earning PP can unlock bonus characters, and there is a specific way to earn PP much faster than the rest.
Arcade mode gives the most PP, but the amount of PP you earn doesn’t go up with difficulty. You can actually earn much more PP than normal by fighting on the easiest difficulty possible, since it’s so easy to do combos. You can earn around 3000 PP per playthrough on the easiest setting in arcade mode, compared to around 2000 on average with the others. And since it’s much easier, you’ll fly through it in around 15 to 20 minutes.
Using this method, you can unlock all characters with about 3 to 4 playthroughs, or about an hour to an hour and a half of play time.