On Level 9, Down with the Ship, you have three minutes to destroy six reactors. Using the Drill Attack, you can destroy a reactor with three hits. Destroy five reactors, then spend the rest of your time fighting enemies. You can earn lots of money destroying enemies. When time is almost up, destroy the final reactor. Go back to Level 2. Purchase power-ups and continues from the merchant then save and quit the game. Load your save, return to Level 9 and repeat the tactic. You can do this as many times as you like and earn oodles of cash in the process.