Par time: 15 Minutes
This Achievement isn't too rough, by knowing the level well and playing on easy I got through this one of my first try in under 10 minutes (9:53) and I even made a few mistakes along the way.
To start I'd recommend playing this on easy. There are a few sections that are troublesome if on higher difficulties that can be run through on easy.
Next, as soon as the opening cut-scene ends start throwing grenades. Keep throwing them until you run out. This forces you to stay on the Pelican that you arrived in during the opening cut-scene. The Pelican will then take off and fly over the jungle eventually "landing" on a ridge closer to the objective. Once the Pelican touches down dismount as normal. It looks a little weird up on the ridge with 2 dimensional trees and a lost/phantom marine if you walk directly away from the tail following the ridge but for our purposes as soon as you get out of the pelican you want to turn around and walk the other way, past its nose. Eventually you'll come to the "A" shaped building that you'd normally enter to progress into the structure. Here you'll jump down and see the covenant running out of the structure as normal but with floating assault rifles behind them spraying bullets. Run through to the elevator and take it down.
Once you reach the bottom you want to head to the only door on this floor. Only waste time killing the covenant that are in your way as the stray fire won't even come close to killing you. Also note that one melee hit is sufficient to kill Jackals on this difficulty so don't waste time trying to shoot the hole in their shields. Once through the first door you'll come into a big room with lots of options. Drop down and head through the door on your right side. Here, like many hallways to follow, the tunnel immediately after the door will split to the right and left. Either path is fine.
Once through you'll come across another group of jackals on your left side. Run past/kill them and you'll come out into another large room. Head over to the right where you should see some standing Covenant energy shields and a few Needlers and Needler ammo clips scattered across the floor. (Fun fact, this is one of the only times in the game that you see stand alone Covenant ammo.) Hug the wall on the right and jump across this small gap to go through the door. After emerging from this tunnel you'll immediately see a marine slouched against the wall firing his pistol at you. Run past/kill him and head to the back left corner of the room. You'll pass a couple dead grunts and some flaming wreckage. Jump up on the wreckage to get to the pathway/ramp above and follow it around. You'll see a door at about one o'clock. Head through and walk to the edge of the platform in the large room beyond. Turn to the left and activate the light bridge, allowing you to cross the room to the door on the far side. If you want to make sure you are in the right room turn around once you have crossed half of the light bridge. You'll see the Needler ammo on the floor below you. continue through the door and hang a left. Grab some ammo on the ground for the Assault Rifle as you follow the ramp down. You'll hear some music as you reach the bottom. Head through the door on your left and skip the cut-scene that follows.
This part "The Flood" is a pain because there isn't really a way to speed it up. You'll need to wait until the flood burst through their doors to progress. First door is at 12 o'clock and contains the infection forms only. *Note* it seems that the quicker you kill the infection forms the quicker the next group will break through. To ensure the forms are dead you may want to physically enter the rooms from which they burst as they will frequently bunch up on the insides of the door jambs. Keep an eye on your radar to check if they've all been killed. With your back to this door the next door they come through will be at roughly 10 o'clock and will again be only infection forms. This is a door that the forms frequently get stuck on. Once they've been killed the next door is at 1 o clock and is the final door with only infection forms. After this door they break through the once from which you first entered and you'll encounter combat forms for the first time. On Easy the pistol rips these guys apart so you'll have no problem getting through this door. Follow the ramp up to the right and take a left as you reach the top to return from where you came.
Now that the Flood are out the doors are unfortunately not the same so escape will take a little longer. As you enter the large room you'll notice a firefight happening below and that the door at the far end of the light bridge is half open and the lights are red. Instead head to the right (staying on the top level) and enter the door there. If you fall down that's ok, just try to avoid the fight and head through the door on the right on the bottom level. Whether top or bottom once you enter the next room your goal is the door on the right side. The one on the top floor is better so try to stay on the top floor. After you head through this door you'll notice another fire fight in progress. Your goal is the door on the right side. There will be some Elite Combat forms around a standing energy shield in front of another door. You'll notice in the corner as you head over that there is a large stack of purple boxes. If you had fallen down you'd use these boxes to get up to the top level to head through this same door.
Once through the door turn left and grab the shotgun off of three dead marines (I'd leave the pistol) and head through the tunnel directly ahead of you. You'll come into a weird room that has a long device with lightning inside. Follow the room to the end and head through the doorway. Here is the first of two elevators. You'll know its the right one because it will be surrounded by dead covenant and have some combat forms milling about. Hit the elevator button and the elevator will go down. On the old graphics you'll see the walls streaked with blood. Not so much on the newer ones. Once the elevator reaches the bottom you'll have to head around the wall to the left and through the doorway. Here you'll find yourself in another large room with a flickering light bridge and two active camouflage pyramids on the right. Grab one, and drop down to head for the door on your left side. You'll notice a standing energy shield has been knocked over near this door. Head through and try to make as much progress as possible while cloaked.
Head for the back left corner of the room and again jump up on the burning wreckage to follow the ramp up to the top level. head for the door at 1 o'clock and you'll find yourself back in the room with the flickering light bridge but this time on the other side. Head directly across and through the door at 12 o'clock. Once you've entered the next room your goal will be a door on the ground floor on the left. drop down and head through. You'll emerge into another big room with combat form flood. Be careful as the Marine forms will occasionally have shotguns. Head directly across and enter the door opposite.
This next room can be a little tricky and eat up some time if you aren't careful. Immediately upon entering the room head to the right. You'll see a small plate in the ground next to two stone pillars on their sides. These pillars will have some dead marines and blue lights on top. Either jump from the small plate or crouch jump up to these pillars. Once on top turn until you see the bent ramp that will allow you to access the top level. Your door is going to be opposite you on the far wall on the top level. Be careful to jump over the glass panes in the ground where possible. The flood have an annoying habit of shooting it out from under you so that you fall down. Once through this door you'll come out into another large room. Quickly activate the light bridge and head across. Combat forms will pour out of this doorway so feel free to take a few seconds and use a few grenades to clear it up. Once through this far door you'll come to another elevator which will take you up. *Side Note* As the elevator starts to rise you'll be able to see a terminal on top of the structure you just came in from, you'd have to jump off the elevator to grab it so skip it for now.
Once the elevator arrives at the top you'll be joined by a company of marines. Grab some shotgun ammo from the area to the left of the door and head out into the jungle. It's easy to get lost out here so stick to the left side of the swamp until the area narrows into a pass. Along the way try to kill as many flood as possible so hurry the marines up. Halfway through this pass flood will jump down and ambush you. Try to kill them quickly and head through to the tower. The Monitor will fly down with some Sentinels and laser the flood. Once all the marines have arrived a second small wave of flood will come. Once they're dead the mission is complete!