This achievement requires that you kill each demon type in the game at least once. Demons are found across Arizona and Oregon, usually in underground locations like the Cemetery Catacombs Fort Ghost, and Sister Ingrid's Mine. Categories of demons include Nightwalkers, Witches, Gargoyle Statues, Female Assassins, Werewolves, and Vampire Cowboys. Within some categories of demons, there are various types that most likely need to be killed for this achievement as well. For example, the Witch category includes both Red and White witches. Based on information from xxKegeR1o1xx, simply completing all the rounds of the Fort Ghost, Cemetery Catacombs, and Save the Princess should be enough to encounter all enemy types and earn you this achievement. The witch missions that require the purchase of the Witch Impaler gun are NOT required.