Revisit area 12 after you've beaten the game once. When at the area just before you encounter the boss a horde of enemies will appear in front of you. Continue to fight them as there will be two waves you'll have to go through. Make sure you have Thanatos in either his adult form or complete form and to also have his healing ability. When your health gets to a low point, summon Thanatos and stand in a corner to heal while having him continuously attacking until your ready to battle again. There are a total of 2104 enemies and it'll take atleast a good hour to go through them all. But if done right you leave with over 13 million exp and as a reward be given 30 exp gems. (take note that if the gems don't appear right away, run back to make sure all enemies are acounted for)