When playing as General Hazel, go to Mt. Machina Kingdom and beat the boss at the end of the stage. Afterwards, you will exit through a door. To your left, there will be two Gold Jewels worth 5000 coins each. Use Hazel's jet-pack to fly over and collect them, then exit through the pipe in the middle of the room. You should warp back into the factory from earlier. Once done, you can complete the stage again, beat the boss, and keep collecting the two Gold Jewels as many times as desired! (Note: Remember to collect the clocks throughout the stage to keep from running out of time)