When you get to level five, go to the Burning Plains Mission (I would suggest that you use Maya, Hyuga and Karen, but any party will do). It doesn’t really matter what type of weapon you use so don’t worry about that. Once you are on the mission, make sure that you purify all the seed and kill all the monsters in under eight minutes. Once you have completed the mission and done everything above, you should get an "S" rating and some valuable items to sell off. You can repeat this as necessary.
Unlockable: Extra Mode
Finish chapter 4 in the Story Mode.
