I found that the best method for earning this trophy is by doing the level "The Battle of Pelennor Fields". If you have all 5 multipliers (x 3,840) and the Attract Studs Red Bricks active you can get anywhere from 290 million to 300 million studs every 2-3 minutes. I also use Legolas for his bow so I can shoot some of the objects that I might miss. There are 4 purple studs in free play in this level that you can collect. The first being during the on rails part of the mission, you'll find it in a bush about 2/3 of the way through the level and the other 3 will be during the boss fight right after the last section around the area. After you get all of these studs Save and Quit and you will have as much as 300 million per play-through.