Successfully complete the game to unlock the Nightmare difficulty.
Proceed through the storyline until you get the chests in the chieftess' hut at Woodburrow that the person refills every day. Then, open the chests, and collect the coins. Go to the "Level Select" screen, and enter a level. Once the level begins, open the menu, and return to Woodburrow. When you enter the town again, the chests will be refilled. Repeat the process as many times as desired to get an unlimited number of coins. You get approximately 440 each time, and it takes 30 to 45 seconds to repeat each time.
Set the difficulty to Hard (or Nightmare if you have unlocked it), and select a bonus mission. When the mission begins, simply change the difficulty to Easy. The clock is longer on harder modes, but when you change the difficulty to Easy, the longer level time will stay and the enemies will become easy. When you finish the bonus mission, the time bonus will also be very high.