Robolifter

On level 8, push two boxes on the left side down to create a bridge to cross over. Push the two boxes under the keycard room to the right to create a bridge to cross over again. On the right side, you should see the secret key card. Go through the invisible wall/floor to get there and pick it up.

Here's a full video walkthrough for all achievements:

Level 1: 0:25
Level 2: 1:10
Level 3: 1:22
Level 4: 1:35
Level 5: 2:02
Level 6: 2:25
Level 7: 2:45
Level 8: 3:12 (Secret Key 1)
Level 9: 4:05
Level 10: 4:35 (Secret Key 2)
Level 11: 5:38 (Secret Key 3)

Achievement Walkthrough: