If you play the game normally you won't get this by accident as there aren't any fights with 100 enemies. Even beating 20 levels of Survival: Horde will only net you around 85. The best place to get this is Chapter 7. As long as you don't destroy the totem the enemies will continue to respawn 3 at a time. Just keep killing them and the achievement will pop once you reach 100. An alternate location would be Chapter 2. As long as you don't close all the cargo containers enemies will continuously respawn. However, the enemies respawn a lot faster in Chapter 7.