After beating the story and watching a cutscene between Dante and Lady, the credits will roll. During this time you and Lady will be fighting of waves of enemies. You will have to Kill 100 enemies to get this achievement, 99 of which will be a mixture of the basic enemies you encountered throughout the game. However the 100th enemy will be a Hell Vanguard and might take some time to kill. Another downside to this achievement is you cant use DT, so having full upgraded devil arms is a definite plus.
You can unlock this on any difficulty, so if you're struggling with this achievement, try doing it as Vergil on an easy.