Arkham Bonus Level
Get a "True Villain" rank on all levels in the Villain storyline. To reach the mission, go to right of the Batcomputer. Go down the ramp, and pull the lever to get to the villains. Go through the door on the right, and look in the corner to reach the memorabilia room. Climb the ladder at the top to access the Arkham level.
Collect coins in a level to completely fill the "True Hero" or "True Villain" bar to get the corresponding rank for that level.