This is story related and cannot be missed.
The challenging part of this mission is waiting for the elevator. If you've played zombie games before, this is where your training ability will come into play. Kinda like cod zombies, run around and zig zag until you're able to line them up then light them up. The main goal is to survive until the elevator opens up. If you're leveled high enough this shouldn't be too hard. If you're under leveled then prepare to die and be overwhelmed. Once the elevator drops down, get in and finish the mission. *Achievement unlocked*.