Select "The Lantern Menace" level in Free Play mode. Go to the first part. Progress from the start until you reach the point where you are facing a large Brainiac skull in a symmetrical room. On the right side should be an electric field, and on the left an electric port. Switch to any character that has an electricity suit (also, put on infinite electricity if desired) and fill the port on the left. This should get rid of the Brainiac tentacles on the top of the stairs, and fire should replace them. Switch to Superman and walk in the middle of the fires. Initiate his freeze-breath. The fire is extinguished, you get studs for it, but the fire will immediately return. Extinguish the fire repeatedly with Superman to get as many studs as desired. Using any stud multiplier will make this even more effective.
