This is near the end of Chapter 14. You will see a generator panel and a generator. Throughout the level you would have gathered 3 circuit pieces. You just need to match them so that the ends with the lights that are on, connect to a end with a light that is turned off. Upon trying to use the Generator, the boss battle will trigger. You can still get this after the boss battle. I believe the achievement doesn't pop until you run the generator. Do not quote m on that.