State of Mind

When you reach Chapter 4 and the storage section you are required to shoot drones in order to help free the robots. There is a separate achievement for saving half of the robots and there is this achievement for saving less than half of them. Shoot a couple of drones and then put your controller down for a couple of minutes while the sequence plays through. At the end Simon will say something like "less than half of the robots got through" and the achievement should pop.

I don't believe this achievement impacts the story so you can safely replay (Load) it straight away to attempt the "success" version of the achievement called "Let my people go".