Aliens Colonial Marines

Story related and cannot be missed. Read below if you wish to know exactly when in the story it will unlock:

This is the final mission, and you are tasked with defeating a Xenomorph Queen. Although she is immune to bullets, shooting her can prove to be an effective diversion, especially in co-op. Don't stay out in the open for too long, as the Queen can kill you in a single hit. You will need to access the four switches on each side of the conveyor belt to pull it further back. Once all four have been activated, head over to the release button and coax the Queen onto the conveyor belt. Providing that you stand right next to the button, you should have adequate protection from her and she shouldn't be able to hit you. Simply press the button to release the cargo and send her flying out of the airlock. Remember to take note of the regular Xenomorphs in the area too, but these shouldn't prove too much of a problem for you.