This one is pretty simple. By default, you start on the UFLL. In their building, they’ll give you a mission. When you head outside, you’ll receive a phone call from one of your buddies. Accept it and you’ll receive a buddy objective, marked blue. Your buddy wants to subvert your current faction’s mission. Simply head to the blue marker and follow the directions, killing all enemies when needed. Once you complete the final objective, you’ll unlock this achievement and the achievement for completing a buddy mission.