From Halopedia, the Halo wiki
This is the second flashback mission that you play from the perspective of Buck. You start out with the usual Assault rifle and Magnum pistol. It's a pretty straightforward mission and you can beat it on legendary with a little creative maneuvering and planning first. Your objective is to escape the city by air. Using your HUD compass, you're going east to west with north/south hallways. Good Skulls for this mission are Famine, Catch, Iron and Cowbell.
But I have My Boarding Pass...
When the mission starts immediately bring up your pistol and take out the wave of Grunts that approach you. Move up and swap your Assault Rifle for a Brute Shot or Carbine. If you keep going straight, alongside the wall on the left, you will find two Beam Rifles. The beam rifles should help you in taking out the Brutes, Jackals, and Grunts. Take out the Plasma Battery and anyone on the lower level, but watch out for the Fuel Rod Gun-wielding Grunt up top.
As soon as he shows up, pop him with the pistol. Start clearing out the opposition, Mickey will help until all the Brutes are dead, then he'll head up to the Phantom. Finish off the Jackals and Grunts here and before you grab the Banshee, swap your Brute Shot for the Fuel Rod Gun. Run over and grab a Banshee, then use it to take out the Shades, there are four of them, before any Covenant can man them. Clear the ground level of enemies then move to the towers. When the door opens take out the two Banshees but before moving forward go to the tower where the Grunts had Fuel Rod Guns and pick up some more ammo. From here you can move forward. Take out the platform and turn north and kill the enemies at the other end of the hall. If you're quick you can get them before they get to the Wraith.
Statistically, still the safest way to travel.
When you enter the next open area, take out the Engineer Recharge Station to the south and deal with the Banshees that appear from the south before you fly up and open the door. Once you open the door, get back in the Banshee and deal with the enemies on the other side. There's another recharge station at the south end, and when you blow it up, it will damage the Phantom hovering above, so you can kill it without worrying about it shooting back. Once you exit there are two AA Wraiths to deal with, kill the one on the east platform to your right before going for the one on the other side. Take out the Banshees that show up from the north, and then clear the towers. Stop to top off your fuel rod ammo on the tower before going forward. Once you get to the door exit the Banshee and head into the inner space.
Long Distance Relationships are Healthy
Inside all the enemies are north of you. Stay clear of the center area and go straight over to the Beam Rifles and swap your Pistol for it. Once you have a Beam Rifle, head up the south end of the room so you have a clear view of the long hall. You'll see lots of Brutes and a few Grunts and Jackals. Pick off the Brutes first with the Beam Rifle, and as the Jackals and Grunts close in, use the Fuel Rod Gun on them to clear out. You'll see an Engineer protecting the Brutes at the back, so you can take it out, making your job a little easier, but you'll probably have to run up and pick up the other Beam Rifle. Watch out for the two Brute Stalkers, then pick off all the Brutes you can see. Eventually you'll draw out the Chieftain with the Gravity Hammer. Hit him with the Beam Rifle until he hits the bottom of the stairs then use the Fuel Rod Gun to finish him off. Once you've killed the Chieftain, run down and swap your Beam Rifle for the Gravity Hammer and move north, clearing out any remaining Brutes. Go to the office at the end on the left and open the door.
Always tip the doorman.
When you run out into the next open space, go south. Run past any enemies on foot and get into one of the two Banshees on the catwalk. These ones are shielded by buildings from the Scarab, so you've got a better shot at taking off without receiving damage. Before dealing with the Scarab, take out all the recharge stations, it clears out all the ground forces that can plink away at you and kill you from behind. Next, deal with the Banshees, watching out for the Scarab. Boosting is your best friend here. Once you've dealt with the Banshees, take out the Scarab. If you can get underneath it, it'll make disabling it a lot easier. Hit its legs, then while it's down blow out the back and take out the core. Once you've beaten the Scarab, open the last door, destroy any of the recharging stations, watch the chain reaction and enjoy the light show.
Notes for Lower Difficulties