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The Ark is a large level composed primarily of large, open outdoor sections and the narrow hallways of Forerunner structures. The level is very vehicle-heavy; in particular, it may contain more Covenant Type-26 Wraiths than any other level in the game.
The Ark begins with a sniping engagement. Players, backed up by ODSTs, should take out the enemies at the first camp as fast as possible. In particular, the Jiralhanae tend to be quite dangerous on higher difficulties if not quickly dispatched, due to their tendency to go beserk and use melee attacks when their armor is destroyed.
After the initial sniping battle, however, the level's gameplay briefly returns to normal—combat conducted on foot, from cover, and with average weapons. Shortly after that, however, the level's large quantities of vehicles come into play. The vehicles on the map are primarily Covenant, with Type-32 Ghosts and Type-25 Choppers being quite common, and the relative lack of cover heavily favors vehicular combat. The assortment of vehicles quickly shifts to favor the heavy vehicles, with Wraiths in particular appearing frequently.
The last stages of the level are conducted inside of Forerunner structures. Individual engagements take place inside of moderately-sized rooms, with long, narrow hallways separating battles. The very last portion of the level involves a small amount of backtracking.
New in The Ark
Installation 00 (Mission Start)
When you exit the Pelican at the beginning of the level, follow the ODSTs to the first Covenant encampment. Hold your fire until you are in position, then snipe out Brute Captain (the one with the Brute Shot). Usually two headshots can kill him, but if you are not sure, hit him in the chest for three times. There's plenty of ammo. Do the same for the others. Continue to snipe Brutes while your allies take out Grunts and Jackals. There are two drop pods in the area with Sniper Rifles: one by your sniping spot, and another by the Deployable Lookout Tower. ODSTs have much longer range than standard Marines, so giving them sniper rifles, Battle Rifles, and Carbines is an excellent tactic. However, do keep at least one long range weapon with standard ammo capacity.
As you move up, a second group of Brutes and Grunts will enter. Some of the Brutes carry additional Brute Shots; these and the possibility of flanking are the only things that threaten your ODSTs. Once the Brutes are out of the way, hopefully with all your ODSTs intact, you can mop up the remaining hostiles easily. Restock on weapons and especially grenades, detach a Plasma Cannon from the tower, and move on through the tunnel.
When you reach the Anti-Aircraft Battery, use a Battle Rifle (if you still have one) to headshot the Carbine Jackals standing on the Anti-Aircraft Battery. Often, the Brutes won't even notice their deaths. Move to the ledge and drop the Plasma Cannon so that it falls on the ground. Switch to your sniper rifle and engage the Brutes. If your ODSTs are along, they should be a big help, though they may have trouble with any Brutes that climb the barricade. Mop up the stragglers with your Battle Rifle.
A Phantom will drop off pair of Hunters (and maybe some Carbine Jackals, who should be priority one). The Plasma Cannon is extremely useful in dispatching the Hunters, though there are plenty of alternative strategies to use against them. Don't be afraid to use close-range attacks—they're slow, and there are plenty of rocks to dodge around. If you're feeling shy, you can either hurl grenades at them from your position above, or you can run around the barricade and try to flank them while the ODSTs distract them. Sometimes they'll come around to try to flank you (though they're easily distracted from this idea), which also presents a moment when their back is exposed. This is one of the few Hunter battles where you don't have any big guns available, so savor it!
Up the pathway is a Brute with a Brute Shot, and a group of Grunts. They are easily dispatched; you need not waste Plasma Cannon rounds on them, and since they drop plenty of plasma grenades, you can lob your own to your heart's content. Meet up with the Marines past the Forerunner door, and ambush the Covenant around the Pelican. The gun on the destroyed Warthog still works. When you get to the crashed Pelican, hurry and grab the Rocket Launcher near its troop bay. Run to the top of the hill, and then wait for a pair of Prowlers to appear right in front of you, and then launch a rocket at each, making sure to lead your targets. Alternatively you can give your rocket launcher to a Marine or ODST, who will blow everything sky-high.
If you want to make absolutely sure that the Prowlers are still usable after the Brutes are dead, you can hide behind a wall or Deployable Cover. Wait until the first Prowler approaches and the Brutes dismount. You can then use the Plasma Cannon you had earlier to mow them down, or rush to the Prowler and man its turret. Don't worry about the second Prowler, as it will usually wait for the first's passengers to die before coming in.
Take your force to the top of the hill and hit the Covenant before they get into their Choppers. There is a little bit of ammo off to the right, and rockets by the downed Longsword. There is a functional Warthog and a Sniper Rifle behind the fighter. As you drive along the path, you will encounter a crashed Phantom, surrounded by Grunts and Plasma Cannons. Past that is a fork in the road. If you turn right immediately, you may skip the next paragraph.
The two main threats in this large basin are the Choppers that drop down from the bridge, and a pair of SpecOps Grunts with Fuel Rod Guns. A good way to deal with these hostiles is to ride around in the basin for a while until the Choppers are taken care of—quick work with a Mongoose and a rocket-wielding Marine—and then retreat a bit and snipe the SpecOps Grunts from across the basin before making your approach. You can pick up the Fuel Rod Guns if you want; they may prove to be useful later. A Shade is fighting Sentinels at the end of the bridge; take it out, and the remaining Covenant forces will be taken out by the Sentinels.
You might want to take a Chopper from here, though a Mongoose/rocket-Marine combo is preferable. The Prowler is another option. A more effective but proportionally more involved strategy would be to give the Fuel Rod Gun to another Marine/ODST, and get your Rocket Marine/ODST and Fuel Rod Gun Marine/ODST to each ride shotgun in a Prowler while a third Marine/ODST mans the mounted cannon. This will take a lot of time swapping weapons and forcing AI allies in and out of particular seats, but it will be worth it. While you lose the ability to inflict damage yourself, you are now a highly mobile and extremely powerful tank, assuming your teammates have any sense of accuracy whatsoever (and the AI usually do). This would also likely be very effective in Co-op as well. What would formerly be a long, drawn-out skirmish with the Anti-Air Wraiths and their infantry/Ghost-mounted backup becomes an instant walk in the park on lower difficulties.
After Sergeant Johnson says his lines, exit the basin and head left, taking out a few Ghosts on the way. Along the way, you can grab the "Famine" skull if you're playing on Normal or higher difficulty. At the top of the next hill you'll meet some more Ghosts and a Brute Captain (who may be wielding a Fuel Rod Gun). They are fighting some Marines, who you may be able to save if you can kill the Brute quickly.
There is a functional Warthog in the area. Get a gunner and rocket-carrying passenger, or use a Prowler if one is still intact. You may use the Chopper, but it is very vulnerable in the next basin. Some players opt to snipe out as many infantry with their Battle Rifle as possible before moving in, and others simply use a Ghost to fire on the Wraith from an extreme distance.
Work on the Wraith and avoid the Deployable Lookout Tower to the left; there are a lot of Brutes with Power Drains there, and there is a Grunt manning a Plasma Cannon on top of the tower itself. As you move out of the cave, two Ghosts will come forward to attack you. This Wraith can be hijacked using a glitch, but the two Anti-Air Wraiths past it cannot.
Once you have dispatched the hostiles in the basin, carefully advance toward the landing zone ahead. Here, two Plasma Turrets (one atop a Deployable Lookout Tower), a few Brutes, and a SpecOps Grunt guard two Anti-air Wraiths. Destroying the Tower should eliminate most of the Covenant opposition and allow you to destroy the two Wraiths from a distance. Once you are done, quickly turn towards the Plasma Cannon (the one hidden in the rocks to the right). To the right of it lies a supply case with a nearly full Fuel Rod Gun.
Forward Unto Dawn (Rally Point Alpha)
When the Forward Unto Dawn lands, take one of the Scorpions that are lowered from its bay. If any ODSTs from previous engagements survived, a few may enter the Pelican and be taken away; make sure that they don't take any useful weapons with them. Go back the way you came and hit the new Covenant forces. This part is absurdly easy; just backtrack while blasting everything in sight—including a large amount of Wraiths, two Phantoms, and a pair of Hunters—until you get back to the bridge. Enter the wall, resisting the temptation to attack 343 Guilty Spark (or not, if you have spare ammo). Activate the energy bridge inside and continue on.
After going through the wall, a Gauss Warthog shows up; let the Marine man the turret. Head right and down to the Scorpions, or go to the left and destroy four Ghosts. You can choose to stay in the Gauss Warthog for a while; they're excellent for destroying smaller vehicles, and it's easy to avoid the Wraith's mortar blasts. Once you've destroyed both lines of Wraiths, a Scarab will show up, at which point a Scorpion is really your best bet (or perhaps even a Wraith if you want to commandeer one). The Scarab itself isn't that deadly; the large metal structure in the middle of the clearing provides sufficient cover, and from its roof, you can attack the Scarab at your leisure. If you want to get the Cowbell skull later on, you should grab one of the Grav Lifts inside of the structure.
Go up the tower; a Brute with a Fuel Rod Gun will greet you. At this point, you probably have a Fuel Rod Gun of your own. You can drive the Scorpion partway up the tower, which makes the approach very easy. If you happen to have a Plasma Cannon, however, you should advance on foot (unless you don't mind backtracking to retrieve it). There are a few clusters of Brutes and Jackal snipers, and once you get halfway up the tower, you will encounter a few Brute Stalkers (who more or less stand around).
Real Men Don't Read Maps (Rally Point Bravo)
Watch for snipers and enemies hiding around corners. Meet up with the Arbiter, 343 Guilty Spark, and two new Marines at the tower, and advance toward a group of sleeping Grunts. Bear in mind that your other Marine allies will bizarrely not follow you to the next section. Give your two new Marines power weapons, such as sniper rifles and rocket launchers. There is a Brute urinating in the corner; he is distracted until you finish assassinating the Grunts, at which point you can beat him to death with no trouble. (If you try to take him out first, he often turns around and wakes up the Grunts). Keep going and assassinate the Grunt wandering under the ramp. Go through the upper door, assassinate the Brute, and melee more Grunts to death. Grab a Plasma Pistol and a Battle Rifle (from a Marine if need be) and move on; alternatively, if you still have a Fuel Rod Gun and a Plasma Cannon, you may keep those. When the door opens, a Brute spots you; greet him with the Noob Combo, and repeat the process for the other Brutes in the room (who are armed with Brute Shots, so stay in cover or flank them through the room on the side). You can also use the Plasma Cannon or just melee them all to save ammo. Kill the Jackals, swap out your Plasma Pistol for a newer one, and move on to the Cartographer.
Gawk at the galaxy in the cutscene, and then head back inside. (Alternatively, if you need a new Plasma Cannon and happen to have a Brute Shot, you can empty it on the side of the attacking Phantom to get a Plasma Cannon.) If you have any Marines left, they might die here unless you gave them some heavy weapons. The noob combo is useful here - you can quickly tear through the defending Brutes that way, before the Plasma Cannon-wielding Brute Chieftain even arrives. The combo also takes out his flunkies, after which he's relatively trivial—stick his cannon with a grenade, or beat him to death, or let your Marine rocket him, or... Next, you'll encounter a large group of Brute Stalkers with Radar Jammers. (You will also see a Brute Chieftain with a Gravity Hammer, but he will leave and let his Stalkers attack you.) The Stalkers are extremely sessile, so you can probably walk right up to them (open doors are a good hint of their proximity) and melee them to death. You might even get Cloaking from one of them.
Also, if you are fast and have a Fuel Rod Gun, you can go right and quickly jump down to the lower level and kill the retreating Chieftain before he finishes exiting. (He is scripted to fall back, so he will not put up a fight.) A decent player should be able to pull it off on Easy, Normal and Heroic. However, this tactic is inadvisable on Legendary as it will leave you in the middle of an empty room with very little cover, surrounded by a fairly large group of cloaked Brutes. Once you have taken everyone out, pick up their Firebomb Grenades and keep going. If you are lucky, you should now have a Plasma Cannon, Fuel Rod Gun, Brute Shot, full grenades, and Cloaking.
The next hallway leads directly to an outdoor area, where a Brute Chieftain sermonizes his pack of Jump Pack Brutes. If, at this point, your Marines are still alive and are wielding power weapons, then they will prove to be extremely helpful; they will often manage to take out all of the normal Brutes as well as their Jackal allies. The Brute Chieftain, however, is another matter. In addition to possessing a Gravity Hammer, this Chieftain also possesses Invincibility, which he will happily update should he start to take too much damage. However, he can still be assassinated (if you have Cloaking on hand, and are careful not to bump into him), and a grenade stuck to his hammer will also easily dispatch him. You can also wait until the invincibility wears off and hit him with a fire grenade.
The last two enemies are a pair of Jackals with Carbines. Go crazy with them, as you will not need any more ammo.
Walkthrough (Co-op with Terminals marked)
The Terminals are listed in this walkthrough.
Installation 00 (Mission Start)
When the ODSTs say "Oorah!", press A. You will jump out of the Pelican and land ahead of the ODSTs. You and your partner(s) should move to the sniping spot, and aim at different Brutes. Fire at the same time. When your enemies are alerted to your presence, they will move more. If you're not a sharpshooter, aim for the Brutes' chests to take down their armor, and then use a burst from your Battle Rifle to finish the job.
When the Covenant pack is down, resupply. Each player should detach a Plasma Cannon from the top of the Deployable Lookout Tower. Walk through the cave, and drop the turrets. Snipe the Brutes on the ground and the Jackal waiting on the Anti-Aircraft Battery. Switch to your Battle Rifle and headshot the Grunts. When most of the enemies have been taken down, a Phantom should drop off a pair of Hunters. Go back for your Plasma Cannons, and use the Cannons to make short work of the Hunters. Move up the ridge, and take out the remaining enemies. Walk into the hall.
First Terminal: Turn to the door on your left, and enter the curved hall. The Terminal should be there.
After you see the Longsword fly to its doom, headshot the two Brutes across the basin. Give your sniping weapons to the Marines. Both players should take out the remaining Grunts, and walk to the crashed Pelican. A pair of Prowlers will enter, so one of the players should grab the rocket launcher from the Pelican, and the other should cover him on the workable Warthog turret. After killing the Brutes, the Prowlers should be intact. Give your launcher to a Marine riding a Mongoose, he'll need it more than you. Mount up; two players should be driving Prowlers.
Drive past the ridge, and let the Marines take out the enemies lurking near the crashed Longsword. There is an intact Warthog with some weapons nearby, including a Sniper Rifle. Give that to a Marine. Make the Mongoose Marines (one has a passenger, one doesn't) use the Warthog. Drive to the bridge, and move up.
Let your guys take out the Choppers, and splatter the two SpecOps Grunts. Take out the Brute too, or else he'll use a Power Drain your Prowler. Take out the remaining enemies.
You may skip the next step by turning right straight away as you take the jump past the crashed Phantom and the Grunts that wait near it. If you decide to deal with that area anyway, simply kill all of the enemies in the basin and get to the top of the bridge. When Johnson's Pelican arrives, drive off the bridge, and don't wait for him. Eliminate the enemies ahead, and go through the cave. Drive toward the Wraith first, and stay very close to it, so that it doesn't disintegrate you with its mortars. Once the Wraith is taken care of, drive left and kill off the infantry near the Deployable Lookout Tower, along with the tower itself and the two Ghosts.
The next part you can do two ways.
If you really hate your enemies, and want to annihilate rather than destroy, then use a glitch to hijack the Anti-Air Wraiths. Now you can seek vengeance with rapid-fire Fuel Rods. If you want to do it the old-fashioned way, then destroy the Wraiths and wait for Commander Keyes to drop off a group of Scorpions.
Forward Unto Dawn (Rally Point Alpha)
Take the Rocket Launchers and Sniper Rifles off your old Marines (so that they don't take them away when they enter the Pelican) and give the weapons to the new Marines. You'll also find a Fuel Rod Gun in a nearby Covenant Supply Case; give that to them as well. Get back to the wall, and use your tanks to easily kill the enemies. Get to the doorway, and enter. Extend the bridge, and turn around.
Second Terminal: After you've activated the bridge, turn around, and walk towards the wall. The Terminal should be there.
Walk outside, and don't be afraid of the Scarab. A Warthog should give you a ride to the bottom. One player should drive it, while the other mans the turret and kills the Ghosts. Link up with the Scorpions, and destroy the Wraiths. Clear out the first and second lines, and wipe out the survivors.
Here comes the Scarab. Drive directly beneath it, where its gun won't reach you (but its legs can—watch out and make sure you don't get stepped on). The player manning the Warthog turret should aim for the knee joints. When it's down, the gunner should dismount, board the Scarab, and destroy the power core. When they jump off the Scarab, the Warthog driver can provide a safe and speedy exit. Drive away quickly, and get to the spire.
The gunner should target the Fuel Rod Brute first, and then the Carbine Jackals (on Heroic and Legendary, they will be Sniper Jackals). Clear the rest of the enemies, and get out of the Warthog.
Real Men Don't Read Maps (Rally Point Bravo)
Go in through the door, and wait for 343 Guilty Spark to open the other one. Kill the sleeping Grunts and the pissing Brute. Go down a level. If you see a Grunt standing next to a lot of plasma batteries, do not shoot the batteries (though that would be fun); whack him. Assassinate some sleepers, and kill the rest.
Third Terminal: Get back to where you first saw the Grunt with the batteries, and walk under the ramp. You'll see a door. Inside that room should be the last terminal on this level.
Go down yet another level, and stick the Brute next to the door. Headshot the rest, and watch out for the one with the Plasma Turret. Just stick him and it's game over for him. After you've dealt with them, keep moving, and activate the Cartographer. After the cutscene, run back inside to avoid getting cut down by the Phantom. If you have a Fuel Rod Gun or Rocket Launcher, you can destroy it by shooting off its turbines. Take out the hostiles inside, descend to the next level, and take out the Brute Stalkers, who should be easily visible thanks to the room's lighting.
Burn and shoot your way down to the level below you. If you have Cloaking, turn it on and assassinate the Chieftain; it'll save a lot of effort. Hammer the rest until they're all dead. Level done. Congratulations, you just beat The Ark.