This achievement will take a bit of planning, and is a somewhat of a pain to get.
Throughout the game, the game will bring you back to the present time, during which you will be talking to Henry Leland about your actions. During these conversations, you need to get Leland to like you. Stick with mainly Professional responses, and when given the chance, try to Bluff him via the Suave stance. Stay away from sarcastic or mocking responses, as otherwise you will not be able to get this ending.
When you finally come to the final mission itself, if Leland likes you enough, he will give you the option of joining Halbech. When you finish off the final boss, you will be talking to Leland near your boat. No matter if you stay loyal or betray him, the achievement will unlock by the end of your conversation with him.