To unlock this achievement, you need to deactivate the 14th orb (the last orb). Don't forget that you need to deactivate the first 13 orbs to unlock the door of this giant orb. To find it quickly, load the 57th checkpoint. When you reach the ladder, instead of climbing down, just go up to a vent. Then, keep going right in the secret room to find the giant orb (watch the video below):
The Last One achievement:Full walkthrough of the game: