This achievement is earned for finding the sixth secret and can be found on Checkpoint 29 or 30
It can also be found in this video, with a text description of its location below
[4:10] - Secret 6 of 13
After jumping over a small ledge, then smashing a red door, you'll enter a room with another red door. Go down and dislodge the crate near the bottom by charging it. Continue down that path into a secret puzzle room. Now you'll need to use the mind controller to put the guy on top of your submarine. Hop back into your submarine with him on top and charge the other mind controller to attach him to it. Go back to the other mind controller with your character to gain control of a new character in the background, and you'll now be able to interact with the secret.