To unlock this achievement, you need to deactivate the 8th orb. To find it quickly, load the 34th checkpoint (when you are about to jump for the mind controlling helmet). The orb is located in a secret underground cave (look for the yellow cable). The cave can only be accessed by removing a plank with the help of your ''followers''. After you go down the ladder in the cave, you need to fight off dogs with a torch to reach the orb (watch the video below):
Pack Mentality achievement:Full walkthrough of the game: