Containment: The Zombie Puzzler

When are about to lose, pause game play and choose to go to the main menu. Select "Continue" and the game will start from the beginning of level you left from. You now have another chance to complete the level.

Play Act 2, Chapter 4 and reach Checkpoint 7. Reactivate the generators and a scene with snipers killing zombies will begin (Screenshot). The kills will count towards the achievement. Reload that checkpoint until the achievement is earned.