This is unlocked for buying all characters out of the store. Most characters cost 1200, while a few cost 1500. There are 8 characters, for a total of 10500 points.
Get points faster:
This seems to be the fastest way i could get credits. Go to Battle Mode, then choose Custom Game. Just put the stock way up, I did 30 and i usually got 1300 points for each match. That being said, you could probably get more than 4000 for doing 100. I found 30 just right. Choose Powerpuff Girls first stage, and as soon as Mojos Robot destroys the other buildings, just keep throwing them off the edge. If you need a Synergy related achievement, such as Johnnys or Bubbles, choose them, and choose either Dexter or Chowder with your second controller (for achievements on using a certain assist against them) or use one of the match-ups listed in "Vengeance of Vilgax" to get the most out of the fights. I recommend saving your credits until youre over 9000 for an achievement, then spend away.
You are awarded:
40 points per Kill
-20 points per Fall
1 point for every energy pickup
4000 pts for a 100 stock game, no deaths, only grabs, 1v1
I suggest holding out on purchasing anything until you unlock the "Its over 9000!" achievement.