Complete the game and load the cleared saved game file, then go to the entrance of the prison and talk to the guard. He will ask if you are a true adventurer. Select the first option and the monster will become stronger. Talk to the guard again and select the second option to return the monsters strength to normal.
Note: Two PSPs with their own copy of the game are required for this trick. Start a Co-op mode game with the other PSP, then complete the game and trade. Send all your items to the other PSP, then quit the game without saving. Your items will all be on the first PSP while their duplicates will be available on the second PSP.
Note: Two PSPs with their own copy of the game are required for this trick. Start a Versus mode game with the other PSP and offer all your money. Intentionally lose and quit without saving the game. You won't lose all your money and the second PSP will still have the amount given.