From Halopedia, the Halo wiki
This is a walkthrough for the level Cortana, structured for play on Legendary difficulty with notes for lower difficulty levels. Cortana is the penultimate level of Halo 3, and arguably one of the most difficult of the franchise. Your mission as stated is to find Cortana on the High Charity, free her from the Gravemind and eliminate the Flood.
Your most valuable pieces of equipment in this level are Deployable Covers and Bubble Shields. Cloaking is also handy but extremely rare. It is recommended that you always have a Deployable Cover with you. Grenades are also your friends here; Plasma Grenades are very effective against Flood Ranged Forms and Flood Carrier Forms, and Fragmentation Grenades will thin out large groups of Flood and bring down shields on any shielded Flood Combat Forms. Beware, though: if you have bad aim, you will be penalized in this level. Flood Blisters line High Charity's walls and if you are inaccurate you will unleash torrents of Pod infectors that will make you waste valuable ammunition and probably kill you. Once you've killed all the Flood in a room, melee their corpses to prevent re-animation of them.
Using Skulls on this level is not a good idea for inexperienced players, but using the Catch skull provides you with a distinct advantage, as the Flood do not use grenades, but upon death, they will almost always provide them with this skull active.
Part 1: Rampant
I'd Say Wipe Your Feet, But... You Know.
You will start this mission with the M90 Shotgun and the MA5C Assault Rifle. At the mission start, use your Assault Rifle to clean out the Infection Forms. Make sure they do not re-animate any of the nearby corpses. Collect some grenades before moving on. Plasma-based weaponry is more effective against shielded Flood, should you encounter any, but for now stick to your current weapons.
As you round the corner, a few Flood Blisters will automatically explode. Eliminate them quickly so that the infection Forms don't re-animate any bodies. Head up the sloped path to reach the chamber's upper level. Find some grenades and replace your Shotgun with an Energy Sword.
When you reach the porta in the floor, drop through it and immediately take cover in the corner behind you. Stay sharp because combat forms with human weapons will appear. Take them down with quick bursts from your Assault Rifle, then close in on them and finish them off with the Energy Sword. Back away slowly and look out for the Carrier Forms that will appear. Use a grenade that sticks (Plasma, Spike or Incendiary) to kill them. When killed with a grenade, Carrier Forms won't unleash Infection Forms.
Where The Hell Did This Come From?
For the next area you'll want to be quick. Follow the path up and around. You should see a couple of Deployable Covers here. Grab one and make your way to the large opening in the wall, leading outside the High Charity. There are many Combat Forms here as well as Stalker Forms that like to become Ranged Forms. Load up on ammunition here at the Pelican and get ready to move. You can grab the Flamethrower here. It can be a great help on lower difficulties, but on Legendary difficulty it is best to leave it: you will probably end up getting killed by burning Flood forms. If you choose to leave it here, it will greatly help at the end of the level (which is at the Pelican). From here you have a choice, you can set up your Deployable Cover and shoot out the Combat and Ranged forms, or you can run past them. You're going to need as much ammunition as possible, so while you could clean out this area (and if you're playing the Meta-Game for points you should) you're much better off just running back inside, going to the left, and making your way to the porta and to the next area. The Flood will not follow you once the porta closes.
The room behind the porta contains a small slope and several Infection Forms. The chamber is filled with dead Elites and Brutes, as well as some grenades. You can throw a grenade to set off a chain reaction and weaken or kill any Flood that re-animates. Finish the Flood off with the Energy Sword. In the next tunnel, grab either a Gravity Hammer (good for clearing away Flood if you get surrounded) or a new Energy Sword (usually the best choice), and then drop through the hole in the floor to continue.
The Control Room. To Your Left, Right, Up And Down, You'll See A Good Chance Of Death
Once you're through the hole, run forward and set up your Deployable Cover. You can shoot through it with your Assault Rifle, and it will provide you with excellent cover from the Ranged Forms. Draw in the Combat and Tank Forms and finish them off with the Energy Sword (remember to lunge at their "mouths"). Once you're down to just Ranged Forms use your Assault Rifle to clear out a path. Run to the right side of the room. From there you should have a good cover and a perch from which you can finish off any remaining Ranged Forms on the ceiling. Switch to your Energy Sword and finish off any remaining Ranged or Tank Forms that may be present on the ground. Now go back to your Deployable Cover at the entrance of the room. When you look to the left of the place where you dropped the cover you will see a Covenant weapons case with a Needler inside. Swap your Assault Rifle for the Needler, then head underneath the platform. Here you can grab a couple Incendiary Grenades, another Deployable Cover, and load up your Needler. The Cortana Terminal is also located here.
Go forward again, around the corner. You will run into two Carrier Forms. Stick the first one with a grenade and let it explode. Don't stick the second Form right away. If you do you will probably end up popping some Growth Pods. Lure the Carrier Form forward and stick a grenade on it at the place where you did so with the first Carrier Form. Take out your Needler and edge forward. Use the Needler in the next long corridor to take out the Ranged Forms. It takes the better part of a caddy to kill them, but it's incredibly fast and effective so it's worth the waste. Run back to the room where the Cortana terminal is to reload the Needler if you need, then head forward again. You will trigger several Combat Forms including several shielded ones. Use what's left of your Needler to take down their shields and finish them off with the Energy Sword. A Bubble Shield is extraordinarily effective here because the Combat Forms won't be able to fire at you, forcing them to move into melee range. Once they're in range of your Energy Sword, they're dead. Once you've cleared the area head back to the Cortana terminal room again and grab a Carbine, a new Energy Sword and the Cloaking equipment. Head forward to the next porta and get ready for another long fight.
Where's A Lift When You Need One?
Once you're through the porta, have your Carbine ready. Zoom in and you should be able to take down many Combat Forms with but one or two shots each. Stalker Forms will attack you here, but you can melee them with a few hits. Once you've cleared as many of them as you can see, reload your Carbine if you wish. Bring up your Energy Sword and activate your Cloaking. Run forward and up the path (it runs counterclockwise for the first part). Eventually you'll run into a few Combat Forms and a Tank Form. Kill them with the Energy Sword and swap your Carbine for a Brute Spiker and Okarda'phaa-pattern plasma rifle combo. This combo will last for a while. Using the cover the first ramp provides you, edge out and use the Plasma Rifle and Brute Spiker to kill the Ranged Forms. You should be able to dispatch most of them from this position before advancing. Once you're fairly certain you're clear, proceed up. It may take a few tries to get the directions and jumps right. On your way up you will run into a few more Combat and Ranged Forms. They will fall quickly under the combined onslaught of the Brute Spiker/Plasma Rifle combo. Grab one of the Cloaking equipments in the room. Once you're through the porta at the top of this room get ready for a run.
Don't Look Down
When the door to the reactor room opens run forward towards the center of the room. You may need to jump over some enemies or crates but go straight to the center. Turn right and run to the wall across the flood grown floor. Turn left and run to the door. Grab a Deployable Cover as you pass through the door. If you got Cloaking, you should be able to cross the reactor room without being seen. If you don't, make a run for it anyway - there's no use fighting the Flood in this room, as more will come in when you kill them. Once the door to the reactor room closes behind you, you won't encounter any more Flood. Run forward to the Sanctum. In the center there will be some sort of podium: Cortana's stasis field. Go to the stasis field and bash on it until it breaks to free Cortana and start a cutscene.
Part 2: Nor Hell a Fury
The Reactor Room, Again
Now you've got Cortana, it's time to get out. You must make your way back to the Pelican. Fortunately, several new areas open up, making the route much shorter. Head back up the long corridor leading to the reactor room. After the cutscene, you only have your Spiker and Energy Sword but no worries, there's plenty of ammunition in the reactor room. Head out and quickly throw your Deployable Cover forward so it's near the edge of the balcony. Back up and use your Spiker to finish off the Combat Forms. This will draw in the Tank Form, which you can finish with one swipe of the Energy Sword. Move forward again and swap the Energy Sword for the Carbine. Take down all the Combat Forms you can see, and then take dual-wield a Plasma rifle with your Spiker. You can take out nearly all the Ranged Forms here quickly with this combo. Load up your Carbine and drop it in the center of the room. Pick up a Brute Shot and a full Energy Sword. Shoot on the four reactors with the Brute Shot to destroy them, and then swap it for the Carbine. Bring up your Energy Sword and run back towards your original entrance.
The Terminal Room
You'll drop back into the long hallway where all the Ranged Forms were. Now there are just a lot of Combat Forms eager to kill you. Use the Carbine until your magazine runs dry, then switch to your Energy Sword to finish them off. Advance cautiously as there are a few Carrier Forms here that will just ruin your day. Stick them with grenades and don't worry about refilling. Once you're back in the terminal room bring up the Carbine. There's not really one good way to pull this off, so use your best judgment for cover and take out all the Flood in this area. Don't try to run through them. There are only about eight or ten Combat Forms here so killing them shouldn't take too long.
After you make your way through this area you'll run into the first large room again. You'll find the Arbiter who will be cleaning out the Flood. If you left the Flamethrower when you first reached the Pelican, the Arbiter will be using it. Run past him and straight to the Pelican. Hop on board and...
Speed Run Walkthrough
If you're having difficulties beating this level the traditional way, you can attempt to adopt a speed run strategy. On Legendary this method is suicide and you won't advance much beyond the Pelican, but on lower difficulties you can make it through nearly every area without a prolonged engagement with the Flood.
Part 1: Rampant
When the level starts, you will drop in with an Assault Rifle and a Shotgun. These will be useful for a short while, but you'll need to switch to other weapons sooner rather than later. Progress through the Infection Forms running, don't waste your ammunition on them. Once you get to the point were the Combat Forms spawn, throw a Frag grenade at them and they should all be dead. Continue running. You should soon get to the point were you drop through the hole in the floor.
When you drop through the first porta you will be assaulted by several Combat Forms. You need to fight these. There is one Combat Form with an Energy Sword, which will definitely come into use a bit later on in the level. When the Carrier Forms attack you, throw a Firebomb grenade to prevent Infection Forms from spawning.
The next room is going to be your fastest skip of the level. There are a few things to note though. To the left side is the opening with the Pelican. There is a Flamethrower and some weapons and ammunition out there, but unless you're already really low on ammo, don't go near it. You should run straight forward, using your Assault Rifle to clear a path to the porta. Once the porta closes the Flood won't follow you.
In the next area there will be several regenerating Combat Forms. Use a grenade and your Energy Sword to make short work of them, then swap your Energy Sword for the Brute Shot (which provides strong melee plus heavy projectile damage). Your weapons should now be an Assault Rifle with about 1/4 of the ammo left and a Brute Shot.
Drop through the hole in the floor. There are loads of Flood here, mostly Stalker Forms that will change into Ranged and Tank Forms, that will all be trying to take you down. Run forward with your Brute Shot out, shooting Ranged forms and meleeing the Tank Forms. Once you reach the center of the room you should get a Cloaking device. Don't activate it yet. Run straight forward out of this room, into the next area. Once you see the Carrier Forms, stick them with grenades. Advance. When you reach the left turn bring up your Assault Rifle and activate your Cloaking. You should be able to make it to the end of the hall without needing to engage, but if your Cloaking fails, the Assault Rifle will make short work of the Combat Forms.
When you go through the porta into the next area, swap your Assault Rifle for a Carbine in the weapons case. Keep the Brute Shot, even if it's out of ammo its melee is worth keeping it for now until you can get more ammo.
Kill the Combat Forms you can see, and melee any Stalker Forms that get close to you. Then run. The path on the far side of the room runs up and counterclockwise. If you have ammo left for your Brute Shot, you are much better off, because you can melee any Combat Form that gets close and a single grenade will kill a Ranged Form with an accurate hit. Grab a Cloaking but don't activate it yet. Just keep running and gunning with the Brute Shot, as it is most effective in this room. There will be a Brute Shot reload at the top of the room just before the porta.
The reactor room is your last challenge here, but just activate your Cloaking at the end of the long empty corridor before the reactor room and run straight forward to the center of the room. Turn right and run to the wall, then turn left and run to the door. You should be able to accomplish this without becoming visible again. Head through the door and down the hallway. Bash the stasis field to free Cortana.
Part 2: Nor Hell a Fury
After the cutscene, head back the way you came. In the reactor room you have no choice but to fight the Flood. Use your Energy Sword to clear out the room. You'll have to be quick, as there are a number of foes here and if you aren't swift you'll be sent back to your checkpoint. Once you've cleared your balcony, swap your Energy sword for a Carbine and snipe any Combat Forms across the room. Then swap the Carbine for a Brute Spiker and finish off any Ranged Forms that remain. Load up your Brute Shot, and grab a Carbine. Use the Brute Shot to blow up the reactors, and run. Fall through the hole in the floor and run down the narrow hallway from earlier using your Carbine and grenades to take out the Flood.
Once you return to the terminal room bring up your Brute Shot and run forward. Toss Frags and use your Brute Shot to clear the area in front of you, and only in front of you. Keep moving forward and don't look back. The Flood will not follow you from one rea to another, so move quickly and you won't need to fight as much.
Eventually you will reach the room where the Pelican is found. You will find that the Arbiter has joined you, with a Flamethrower. Just let him do the killing, and run past all of the Flood to the Pelican. Go into the Pelican and escape.
Notes for Legendary Play
Tips & Tricks
Activating Skulls on this level is at your own Peril, but here is a breakdown of skull use on this level and how it will affect your gameplay.
No HUD, you cannot see ammo, grenades crosshairs or guns and no score addition. Not suggested.
Bigger explosions. Stay away from grenades you toss, carrier forms, and any other explosions. Useful.
No effect on this level. Use anyway.
No Grunts on this level. Useless.
Not Legendary notes