If you fail a stage, you will start again from the beginning but retain any experience and money that you have earned. To take advantage of this, play a stage where there are lots of enemies or bosses to destroy. Destroy all but one enemy unit (preferably a Boss unit to prevent clearing the stage), then intentionally end the stage in failure (destroying all of your own units with MAP weapons, killing a certain character in that stage, etc.). You will then be taken back to the Intermission menu with all the money and experience you have earned.