At the very start of the level, hold your action/reload button to get out of the Pelican as soon as possible. There are some jumps that can be done on the cliff to the left to cut corners as you approach the first encounter. When you reach the first enemies, jump onto the wall from the rock to the far right. Immediately snipe the 2 Brutes in the center of the area and keep walking to the cave. Grab the Deployable Cover or Flare if either was dropped.
In the second area, if you have a DC or Flare, jump onto the Covenant crate at the cave's exit, grenade jump, then equipment jump on top of the cliff to the left. If you did not get either equipment, continue forward, hugging the left cliff, until you reach the support beam. Jump onto it, then turn left and jump onto the cliff. You need forward momentum for the jump, so take a step or two before jumping. When doing this jump, you need to walk along the edge of the cliff for a bit to avoid hitting a very long string of loadzones that causes a lot of lag. Continue to the hallway, ignoring the Grunts and the Brute.
In the Mongoose area, snipe the Brute that is straight ahead near the Mongooses. Jump down to the right of the crashed Pelican and swap your BR for the Rockets on the ground. Make sure you picked up the rocket crate that is right next to it (you should have 4 rockets). Take a Mongoose and drive up the hill. Stay to the right as you approach the jump to avoid getting hit by the incoming Prowlers. Do not hit the tree as you jump. Continue driving through the middle of the area between the 2 rocks and take the nearest Ghost before the Grunt reaches it. If you mess up, you can keep going and take the 2nd Ghost farther back. Boost through the mission until you reach the AA Wraith area.
When you reach the cave just before the AA Wraith area, get out of the Ghost, pull out your Sniper, and zoom towards the rock structure between you and the AA Wraiths. You can barely see the legs of a Brute there. Move your reticule onto the Brute's leg, switch to your Rockets without moving, and fire. This will clear all the enemies there. From now on, keep the Rockets with you, as they are needed at the end. Now you can get back into the Ghost and boost to the Wraith. Destroy it via boarding, then reenter the Ghost. Shoot the Grunt in the tower near the AA Wraiths with your Ghost, then kill the drivers of the 2 other Ghosts. Finally, destroy the AA Wraiths by shooting the circular vent in the back with the Ghost. At this point, the Forward Unto Dawn is on its way. While you're waiting, swap you Sniper for the Fuel Rod Gun in the ammo crate. Get your Ghost again, and wait in the rock structure. Make sure you don't get pushed backward as FUD arrives. You will be forcibly ejected from the Ghost; get back in. Now, wait just to the right of the rock structure until the cue for the next chapter appears ("Forward Unto Dawn").
Boost to the cave from which you entered, avoiding the Ghosts and the Prowler that come out. Continue past everything until you reach the large area with the bridge. Here, you need to hug the left side of the area and head towards the nearest Wraith. Take it and use it to destroy the other one nearby. Keep moving to the bridge, and kill every enemy on it with your Wraith. Once you reach the closed door at the end, get out and take the nearest Chopper parked on the bridge. Wait in front of the door until it opens (this can take quite a while). You can barely drive the Chopper through the door. Drive to the end of the hallway and turn your camera into the back right corner for a few seconds. This spawns 343 GS to open the door for you (the game will not spawn him if you are looking at the door). Keep driving your Chopper through, get out, push the button, and meander through the next 2 rooms to the next area outside.
As you exit the structure, immediately drive as fast as you can straight to the flat area to the back of the level. It is important that you don't flip along the way. Drive into the little building in the middle of the flat area. Slow down as you take the jump to avoid letting the Chopper go too far and dismount it. Grab a Grav Lift to your right and drive to the back right of the area. Throw down your Grav Lift (see vid for specific location) and jump into it the moment the Scarab hits the ground. You need to wait a few seconds before destroying the core with your FRG. All of this needs to be done before the Scarab spawns and jumps down. In the case that you flipped or something on your way to the Scarab, you can opt to skip getting the Grav Lift to save enough time to FRG jump onto the Scarab instead. If you still do not have enough time, you can grab a Grav Lift, drive on top of the building, place it on top, and drive your Chopper over it to jump onto the Scarab.
Once the Scarab is destroyed, get back your Chopper and drive up the ramp to the right side of the flat area. The Chopper is no longer needed. You need to kill at least 2 Carbine-wielding Jackals and reach the top of the ramp to trigger the Pelican to come. While you're waiting for it, go back down the ramp a little and see if you can obtain a Deployable Cover from one of the 3 Brutes there. It is not absolutely necessary, but it saves a fair bit of time later. When the Pelican arrives, wait for Arby and Marines to get out and throw a plasma grenade at them. You want to kill both Marines in order to skip some dialogue while waiting for Guilty Spark to open the door inside the structure.
Run through the first room without waking the sleeping Grunts. You can do a small slide jump in the next hallway, but make sure to jump a little down the ramp to avoid hitting the ceiling. Stay to your right in the next room, then drop down through the beams and descend the staircase. Go to the door to the back left of the Brute room. If you did not obtain a DC before, now is a good chance to get it. Keep going and activate the cartographer.
Turn left after the cutscene and run back to the door you came from. Walk straight ahead to the door near the Jackals and avoid the Brutes in the staircase there. In the next room, you need to quickly kill the Chieftain by firing three fuel rods before he activates his Invincibility. Drop down to the door. Turn around and block it with your DC after walking through. This blocks all the Stalker Brutes and allows you to skip killing them (the door locks later). Sometimes, one does not despawn and makes the level impossible to complete, but that is very, very rare. Kill the 2 Jackals with your FRG and descend to the last area. Now, pull out the Rockets you've been carrying this whole time. Fire the first rocket between the 2 rightmost Brutes. Fire the second between the next 2. Switch to your FRG and kill the last 2. Quickly turn around and try to throw a grenade at the 2 Jackals that spawn in the back. If you miss, you have to get to them and kill them yourself. Once all the enemies are dead, you'll hear Johnson say he'll be there "damn quick." Walk to the back of the area and wait near the edge for the Pelican. Do not stand right on top of the edge; stay a few steps back. Otherwise, you miss the trigger to end the level.