"Disciplinarian"
Early in Ch. 1, Mors will confront his four recruits for a training session. When you enter the training area, a couple of your recruits will be bickering between themselves. You will then be prompted with a dialogue choice. In order to pop this achievement, you need to choose: "I'll show them who's in charge here." You will then need to win your first two one-on-one fights. Once all the fights are over and all the dialogue is finished, the achievement will finally pop.