Find the Crashed Flood Ship

A Floodship has crashed nearby, unleashing this ancient plague on the city. You'll have to backtrack through the level you ran through previously, fighting them off the entire way. However, because most of the Flood are unarmed, you can actually just run through most of this level without firing a single bullet. Our walkthrough is actually designed around this method, but if you really want to kill everything, the best strategy is to just stay as far back from the Flood as possible (such as by the entrance of each room) and blast them from afar.

Work your way down the hill, toward the warehouses. Shortly after a panicked soldier radios in, you'll encounter a small squad on a ramp to the left being ambushed by a small army of Flood. Don't bother trying to save the soldiers - just run into the open door on the right side of the ramp and continue through the hall. Drop off the opening on the left side into the first warehouse.

Run across the warehouse, jumping over any Flood in the way, to the opening in the northeast corner (on the right), leading to the second section.

Run to another opening on the opposite side, in the southwest corner, leading to the third section.

Run straight ahead, under the bridge, and jump from one of the boxes to a door on the walkway, and drop through the hole within.

You'll drop into a small room - smash the glass and step into the main room. Swap out your Assault Rifle for a Shotgun off of the gun rack, next to the hallway entrance. Now get ready, as you'll have to battle a few foes soon.

Step into the hallway with shotgun equipped, and toss a grenade as you turn the first corner to take down most of the Flood here. Immediately backup, and take down any that give chase with your shotgun. Once the hall is clear, continue through a series of rooms until you emerge on a walkway outside.

As you'll see, a ton of Flood will drop in on the area below. Don't worry about them - stay on the walkway and follow it to the end. Once there, drop off the left side to the path below and slowly push forward.

Several Flood will soon jump in from the left. Hang back and use the boulders for cover as you shoot the Flood beyond. If you need additional protection, look for a Bubble Shield near a rock in the center of the path.

Continue up to the ramp, and use the boxes for cover and shoot a few more enemies that drop in. Then climb it and enter the building on the right.

Inside, several more Flood will appear inside the tunnel. Hang back and use the left side for cover as you target the enemies beyond (launch a grenade or two if you have them available.) Push through the tunnel when clear.

As soon as you enter the next room, jump onto the walkway on the left and follow it up to a slit in the wall, which you should drop through. After dropping, grab the shield regenerator and toss it down, as a whole bunch of Flood are about to drop in. Once the shield depletes, take cover and blast the rest. Once clear, head outside.

Search the Ship for Cortana

Back outside, take control of the turret on the left and open fire on the Flood running in ahead. Stay here until they're all down, then follow the walkway slowly. Take your time to target the enemies, one by one. Make sure to take out a pair of snipers on a platform, left of the walkway.

When at the walkway's end, drop into the glowing ship's debris below, and climb up the steep wreckage and drop into a hole at the top.

Work your way through the halls, down another hole, until you come across a glowing blue object. Interact with it to complete the mission