Play the mission with two players under the Legendary difficulty setting. Skip to Rally Point Bravo and continue until Dr. Halsey announces that her lab is open. Do not enter the lab or use the panel on the door to end the mission. Get an unmarked Jet Pack from the wall at the northern side of the lab entrance area. Have one player remain in a safe location and have the other player fly to the far eastern line of buildings. Run up the ramp and turn left (north). Notice the fenced-off area on the upper level lines the out-of-bounds region to the northeast. Drop off the northeastern corner and carefully move towards the rocky out-of-bounds area below. Do not touch it. Hover above it and search for a panel with a green light which you can access on a girder. Press X to activate it. This opens a door on the north side of Halsey's lab entrance. Some Elites with Energy Swords will appear from there. Fly over them or run a Ghost through them, then enter the open door. You will be transported to a hallway. Follow the hallway to the lab area, where you can see hidden features including old Halo artwork and data panels with joke text.