Work your way through the level called The Lantern Menace until you reach a machine where Batman or Cyborg must provide an electrical charge. Do as instructed and a step should appear, along with a fire. Now, equip Batman's ice suit and try to put out the fire. It doesn't ever extinguish entirely, and studs should keep appearing to reward your efforts. You can continue producing studs in this fashion, which makes it easy to collect millions or billions of studs in a relatively short period of time (especially if you have multipliers enabled).