Part 1: Once More Unto the Breach
As we start the mission, continue straight ahead and down the ramp at the end of the corridor. Head over to the marked pelican and hop on board. The lift will lower. When we regain control we’ll be in control of the pelican. Hit the boost button to fly out of launch bay and out of the ship.
Fly towards the glowing sphere in the distance. As you approach, a pair of objective markers will appear. Let’s fly over to the right hand marker first. Destroy the phantoms lurking around outside the objective before bringing the pelican into land. Hop out and enter the door.
Once inside the building, follow the long hallway to the end and through the door into the next room. Kill the pair of watchers immediately inside the door. Look over the edge to the right to see another pair of watchers. Kill them too. Look over to the left for yet another two watchers. From the platform here, try to eliminate as many enemies as you can. Head over to the objective marker and hit the button here.
Turn left and return back the way we came. Kill the watcher, knight and group of crawlers that have now appeared. Return back to the platform in the middle and continue across to the opposing side of the room. When you reach the far platform, turn to the right and follow the bridge here. As with the previous section, clear the platform at the end of enemies, a knight will spawn as you approach. Kill him and then turn left and destroy the exposed power node. A knight will appear behind you again so kill him and return up the ramp we just came down immediately before reaching this platform.
Kill the watcher, knight and group of crawlers that have now appeared. Instead of returning to the center, continue straight along the walkway and turn left. Defeat the watchers, knights and crawlers on the platform here. Once it is clear, cruise to the very back of the room and destroy the power core here. Return back to the door we entered from. In the long hallway, as you approach the exit a knight and a few crawlers will spawn. Kill them and return outside.
Jump back into the pelican and fly over to the other objective marker. Kill the phantoms outside again before landing and continuing inside. Inside, head down the ramp to the left and continue into the gondola ahead (Halo 2 anyone?). We are going to ride the gondola to the objective on the very far side of the room. However, if you look ahead you will see two structures en route. Unfortunately the gondola is going to be stopped at both of these.
At the second stop, as with the first you’ll need to head across the Lightbridge to the structure, work your way through the various groups of crawlers, watchers and knights until you reach the top platform with the button to re-activate the gondola. Kill the enemies up here and hit the button to re-activate the gondola. Cruise over to the new Lightbridge killing any crawlers along the way and then take down the two knights who have snuck onto the gondola whilst you were out and you will notice that the gondola is now giving you access to the power source.
Continue over to this, killing the knights as you go. And go and stand in the yellow beam for a few seconds. Kill the large group of watchers that appear to attack you, return to the gondola and hit the switch up on the top deck to have it return to the far side of the room. When it reaches the end, return back to the door we entered from. In the long hallway, as you approach the exit a knight and a few crawlers will spawn. Kill them and return outside.
Hop back in the pelican and after a few moments, Cortana will give you a new waypoint to head for. Land on the platform that appears, land and head inside.
Part 2: Change of Plans
Continue onwards to the next platform and kill the elite and group of grunts inhabiting it. Finally make your way over to the platform with the blue gravity lift on it. As you approach, a pair of hunters will drop down to attack you and the blue beam will disappear leaving you with the necessity to kill them.
Note: At this point, you can try for this mission’s specific achievement 'Explore the Floor'. If you can lure one of the hunters to fall to its death, you will earn the achievement. This is easy if you have a jetpack!
Once the hunters are dead, enter the lift and fly up to the next area. Head across the first light bridge and kill the elite here. Now you can either grab a banshee from here and use it to boost all the way across the room, dropping down to the door when you are close, or you can slowly fight your way through the room, killing enemies whilst dealing with banshees as well. I tried both and the banshee boost is so much more efficient!
When you reach the door at the far end, climb the ramp behind and use the terminal in the next room. Grab Cortana back out of the terminal when necessary and when the platform lowers, run and jump off of the cliff in front to complete the mission.