Defeat a total of 10,000 enemies.

This and "Proud Millionaire" will be the last achievements you get in the game. There is no way to keep track of how many kills you get in game, so all you really can do is kill, kill, and kill. If your trying to get "Proud Millionaire" as well as this, your best bet would be the Bloody Palace, as each playthrough will get you anywhere from 650-750 kills. Another good place is to go to Mission 6 on Human mode. During the fight with Agnus, infinite Gladiuses appear until you destroy the glass. Instead of throwing the Gladiuses, snatch them, and quickly kill them. Repeat this to your heart's content, and then finish the level.