Complete the game and then select the "Next New Game" option to start a new game with all previously earned experience, skills, parts, and money. The "Sound Test" option will also become available. Additionally, depending on the number of the times the game has been completed and saved, the enemy HP is multiplied by the following amounts:

DIFFICULTY - ENEMY HP
x0.8 and x1.2 difficulties - One time
x0.5 and x1.5 difficulties - Two times
x2.0 difficulty - Three times
x5.0 difficulty - Four times
x10.0 difficulty - Five times