From Halopedia, the Halo wiki
This is a walkthrough for the Halo 4 campaign level Forerunner. It is the third playable mission. This walkthrough is written for play at the Legendary difficulty all strengths of enemies reflected are at the Legendary level, but contains notes for lower difficulties. Like the previous level, you do not have any allies who accompany you. Your starting Weapons for this mission are your Trusty Assault Rifle and Magnum Pistol. Your Starting equipment is the Hardlight Shield.
Buried & Forgotten
What We Have Here is a Failure To Communicate
So the Slipspace Portal has delivered you somewhere. Look to the sides of the hallway and swap out your hardlight shield for an Auto Sentry. Advance forward. You'll arrive at a console that you'll insert Cortana into and she'll tell you that you need to deactivate some power lines to Communicate with Infinity. Hop through the Portal that appears and get ready to fight. As you come out of your little start area you'll encounter a few crawlers that run away. Don't waste your ammo here, wait for the real fight. Advance again and your going to encounter a pack of Crawlers. They'll fall easy enough to a headshot so use your magnum and dispatch them quickly. Move again and activate the light bridge. Cross it and you'll encounter your first Knight in a similar fashion to the Sangheili from Dawn. Watch him run away and advance again. You're going to encounter another pack of Crawlers, a Watcher and a Knight. Deploy your Auto Sentry here. It's very good at keeping the watcher out of your hair while you deal with the Crawlers. Kill them with headshots then redeploy your Auto Sentry and finish off the Watcher. With any luck it will only take a single burst from your Assault Rifle thanks to the Auto Sentry. Now the Knight, on Legendary these guys are hella tough. they regenerate shields quickly, they teleport out of your line of fire and they close really fast and can melee you to death before you can think about it (And if you don't kill the watcher first they get to have a hardlight shield and someone to resurrect them after they die). Use the Auto Sentry to distract and short bursts to kill him. Swap out your Assault rifle for the light rifle he dropped. It's similar to a BR85 Heavy Barrel Service Rifle/M395 Designated Marksman Rifle combo. It has the three round burst when not zoomed and a single high power shot when zoomed in. Advance into the structure. If you don't have any ammo left for your Magnum (and you're not trying for the This is my Rifle, this is my Gun achievement) grab a suppressor off the ammo crate on the wall. Advance again and you'll encounter another Knight. Deploy your Auto Sentry, close and using the suppressor in short bursts take it out. If you go to the right you'll encounter another Knight several Crawlers and a Watcher. If you go straight to the left you'll skip them and save some time and ammo. Stay left and advance to another platform. Several Crawlers will jump up onto the platform, kill them with head shots then look to the far right wall, you'll see some more crawlers climbing here. Headshot them then refill your lightrifle from the crate on the right hand side. Look down and forward and you'll see a Knight Battlewagon deploying a Watcher. Take out the Watcher then deploy your Auto Sentry and take out the Knight when he jumps up to the platform. Finish off any crawlers you see below and swap your suppressor for the Knight's Scattershot. Advance into the hallway. On the left there is an alcove with reloads for your Lightrifle and Scattershot.
Exit Light, Enter Knight
Advance again to the door but do not go through yet. There is a large Forerunner structure. To the right side you should see a Knight deploying a Watcher. Kill the Watcher then the Knight using your Lightrifle and Auto Sentry. Run back to the alcove and refill your ammo and Grenades if you need. Once your full up advance out of the hall but don't leave the ledge. Kill all the crawlers you can see, there are plenty around, and if you can spot the Watchers kill them too. Once everything you can see is dead drop down and go to the right side of the structure. You will be directed to destroy three power spheres that control the shields to the place you need to go. Kill the first one with a melee then look out the other side you should be able to see the second one well enough to shoot it with the Lightrifle. Once the second one is dead finish off the crawlers and the Knight. Before moving to station three, finish off the remaining ground floor enemies. There are a few crawlers a Knight and a Watcher. Now take a minute and go into the central structure. Using the Scattershot clear out the Knight Battlewagon on top. Reload your weapons and go back to the entrance to the central structure. On the opposite side to the power core you need to destroy deploy your Auto sentry. Now run to the core and blow it up. Two Knights will appear at the entrance to the central tower. If you were quick enough you will have enough time for the Auto sentry to distract them while you sneak behind them up the tower to the Pylon. deactivate the Pylon then jump through the Portal.
Enemy of My Enemy
So Many Targets. So Little Time
Now you're through the second portal and onto the next area. Drop down into the hole and advance into the next area. You'll see a whole lot of Covies fighting Prometheans. Sit back for a few and watch the carnage. Eventually there will only be a few troops in this area to mop up. Most likely a Sangheili and a couple of Grunts. After these easy pickings move on the right side slowly and carefully once you see the next battle pick off the Watchers, then the Knight Battlewagon (who should be easily handled being weakened by the Elites pounding on it. Finish off the Elites, then move up. You'll see a Knight Battlewagon eviscerate an Elite. Using your Scattershot you should be able to finish it off fairly quickly, before the Watchers get involved. Now take out the three Watchers that are left and a Beam turret. Once done advance again, you'll encounter several Sangheli on Ghosts that are killed by a Knight. Sneak up behind the Knight after and assassinate him. Run back and grab a Ghost and slowly make your way forward. Sweep through the area and kill all the Covenant troops here. Keep your Ghost and move on to the next area which contains Crawlers fighting Unggoy and Kig-Yar. Move into the structure. Grab any reloads you need before heading out.
They Don't Yield for Pedestrians on Requiem
As you exit the hall you'll see a Sangheili and a Banshee to the left. Splatter the Elite and leave the Banshee for now. Don't stop moving. Circle the structure splattering and shooting as many Grunts and Elites as possible without getting killed. There are two more Banshees in the air and a ghost on the ground just avoid them as best you can for now. Once you've splattered a majority of the ground forces kill the Ghost. Now head back and grab the Banshee. You should be able to kill the flying enemies now without having to worry about ground fire. Once they're destroyed focus on the central structure. There are a few Elites there, Drop some Banshee bombs on them and finish them off with the autocannons. Land on top of the tower and hop out. Mop up any reamining ground enemies, then take out the shield power cores. Run back up the tower to the pylon and deactivate. Jump through the Portal.
If you are unsure of your piloting skills with a Ghost you can try this method. Splatter the Elite when you exit the hall. Boost your Ghost to the entrance to the Central structure and up the ramp. get off and clear out the Elites. Once you're clear up top start using your Lightrifle to clear the ground. There are a couple of Ammo crates up there for reloads. Be wary of the Banshees, get under cover when they start to strafe you. When all the ground enemies are dead, hop down and kill the power cores, then run up and deactivate the pylon.
Once you're through the third portal, sight down your Lightrifle this is the last shooting you're going to be doing for the whole level. Take out the Grunts and Jackals at the end of the room then turn left. Open the door, now Run. Head up the left hand structure and assassinate the Elite here. Now run to the back of the area into the door that opens. The ledge at the back of the room goes up and down so get on and go up and out the other door. Repeat the movement of the last area and go left, only this time don't kill anyone, just keep running, they're all too focused on killing each other that unless you give them a reason, they'll pretty much ignore you. Hop up the back past the Knight, and up the ramp. Keep going running and jumping and avoiding being shot by the Beam turret and hit the last activation area. Boom.
Next Stop, Certain Death
And Yet Again, the Floor Falls Out Under Me
Hop on a Ghost and drive. Cortana is going to boost you, and you don't have weapons, but don't worry, no one is going to shoot at you. Just keep going until you hit the portal. Watch the cutscene, you're done.