In order to earn this achievement, you must kill at least five-hundred enemies throughout the course of your playthrough.
This can be easily earned before the end of the game. You can either find an area with a high amount of monsters in the area and clear it out before finding a save point, pretending to change your costume, then back out so that the entire area can respawn. Or you can do what I did and just play through the game normally, taking out one monster after the next, minor or major foe. It doesn't matter who you take on, as long as you kill five-hundred monsters, you will earn the achievement.
Once you earned your 500th monster kill, the achievement will be yours to unlock. Have fun and good luck!