MISSION 15 - Guardians
Objective: Reach the cryptum
When you gain control at the start of the final mission, stroll down the corridor in front of you towards the large glowing orange sphere. On a ledge to the right of this sphere is a forerunner console you can access for intel. Another console will be to the left from the first one so make sure you pull the intel from that as well. Exuberant will talk to you for a while before opening a door on the other side of the orange orb. You'll walk out onto a bridge with the skies above full of guardians.
Step onto the elevator at the end of the walkway. The vanishing guardian to your left will inflict some shield damage but not actually hurt you. After it finishes disappearing, the elevator will begin to move across the gap. Enter the new hallway on the other side when you reach it to enter a rocky ice covered canyon beyond.
You'll enter a narrow cave at the end which opens up almost immediately into a large open area ahead of you. A group of three grunts and an elite will be visible directly in front of you, but the area ahead is packed with covenant and promethean enemies battling against each other, so be careful.
Note: Your AI allies may glitch here, remaining in the cave even though you move forward. You'll need to move further forward for them to move out of it.
More elites will come into view in the covenant line ahead, and further past them on a raised platform in the middle of the area will be a pair of promethean sniper soldiers firing down on them. You're basically going to want to perform a pincer attack on the covenant forces here, using your magnum to pick off the grunts in the area. Then take cover and start firing on the three or so elites caught between you and the snipers (the elites shouldn't be as effective at dodging when under fire from two directions). A couple of jackals will be skulking around the burning wreckage with the elites, so bring them down afterwards, avoiding the shots from the snipers above.
Use your battle rifle to quickly bring down the soldiers and take the time to observe what is fighting ahead. There's a hunter battling away to the left, who will open fire if you simply run out into the central area below the central platform where the snipers were. There is a light rifle and incineration cannon leaning against a wall here however that you will likely want to collect later. Instead, jump up onto the snowy ledge that runs along the right cliff-side. You should find a plasma caster in the snow up here that will help you stand a chance against the hunters (yeah, they always travel in pairs).
Swap it in to replace your magnum and you'll likely see the second hunter and an elite or two at the back of the area ahead. When you reach the back half of the area, promethean soldiers will teleport in onto the ledges and main section of the back of the area engaging the hunters. At least one hunter should have it's back to you so while both sides are distracted, use your plasma caster to bring down one of the hunters. This will most likely empty your caster, so backtrack to the wall I mentioned earlier and pick up the incineration cannon to replace it.
Now you'll want to eliminate any remaining elites if you can before taking on the second hunter (as it's easier to dodge just the hunter's shots). Empty the incineration cannon on the hunter (it likely won't kill him but will deal significant damage), then go back and pick up the light rifle to replace the empty cannon. A splinter turret soldier will probably be drawing the hunter and any remaining enemy attention, so shoot at both sides, leaving the turret user alive until the end so he continues to weaken and hopefully kills the hunter (you can help by shooting it in the back and legs). Polish off the turret user to finally clear the first area. Pick up any fallen light rifle ammo you can find and a binary rifle dropped by one of the snipers on the raised platform in the center of the area as well as a data pad with intel or a fuel rod cannon dropped by an elite before moving up to the door, picking up the splinter turret on the way.
A mass of six or seven crawlers will be waiting just ahead, and as you approach a watcher and a couple of soldiers will warp in as well. Start using your turret to quickly put them down, You'll see a pair of weapon stands directly in front of you that hold ammo for your light rifle, so make sure to fill up before moving forward. A knight will be up on a ledge along the left cliff face, but you'll want to get rid of any remaining watchers before bothering to deal with him. Just make sure to avoid the knight's shots as you move around killing watchers off.
More soldiers will also be along the canyon floor here, so be careful of their rapid teleports as they can suddenly be very close to you. A pair of turrets at the back of the area will start firing down at you if you get too close, so fire at them using the binary rifle I recommended you grab earlier to blow them up fast. They should take about three shots (you may have to finish off the second with your light rifle). When all of this is done, you'll finally be able to take on the knight that has been annoying you. Swap out your empty binary rifle for a boltshot and to restock your light rifle.
You'll have to be wary of the blasts from the enemy knight, but by now you should be well practiced at taking these guys down. Order your squad to attack while you try to pick off the armour and nail the shot down the throat. If that fails, you'll need to run up the ramp to take him on (hopefully your squad will distract him but I wouldn't hold my breath). When the knight finally goes down, the area should be clear. Restock your light rifle ammo, take the knight's incineration cannon then trundle up to the back of the area to meet up with Exuberant. As the door opens, there will be a couple of racks you can replenish/ grab a light rifle from on the left. You can take the binary rifle on the right as well if you don't want to use an incineration cannon, either one will work fine.
Objective: Destroy the cores
In front of you at the bottom of the slope you stand atop will be a ring of metal walls housing a core. You will have to destroy five of these damn things to continue your search for Master Chief, but each one is heavily guarded. As you reach the bottom of the slope a covenant phantom will arrive and drop some elites, grunts a ghost and jackals off. At the same time promethean soldiers will warp in and start attacking. Take cover against one of the big circling walls to avoid the plasma shots from the drop-ship, waiting until it departs to engage the enemies.
It shouldn't take you long to chew through the covenant enemies with your promethean light rifle (takes three shots to kill an elite on legendary) so put them down. The main issue is going to be the ghost swooping around at the back of the area. Get your squad to target it when everything else is dead an their hail of gunfire should take it down pretty fast. When the coast is clear, you'll be left with a core spinning away in the center of the area. If it's still in one piece you can flip the ghost and use that to destroy the core without wasting your own ammo, or just shoot it.
When a core explodes from now on is the only time you'll get a checkpoint, so be prepared to replay this area a lot if you don't play defensively. Look at the downed wreckage from a covenant vehicle against the cliff wall to the left to see a carbine and plasma caster which you should conserve for use against hunters later on. Note that you can do each core in whatever order you like, as you will need to do all of them to move on. A data pad next to these weapons holds some intel for you to grab. There are also mantises and several ghosts you can commandeer in this area, so take advantage of these powerful vehicles to increase your odds of success. Head towards the main area where the majority of cores are located, looking for a tunnel to the right that leads to gamma core. Climb onto a ledge on the left side of this tunnel to find a broken constructor with some intel. From here you can continue on to gamma core, where you'll find a downed pelican. On a ledge to the right of the pelican is a data pad with intel on it. On the other side of the downed pelican (a couple of meters away) is the final skull on a ledge to the left of the pelican. Head for delta core and travel into the tunnel to your left. You'll find a data pad with intel behind a ghost. At the entrance to this tunnel on a ledge above is a downed constructor you can scan for more intel. In between epsilon and delta cores look for a ledge above in the rocks. Climb up to the high ledge to find a downed constructor you can pull intel off. Close to Epsilon core, there is a sloping ledge leading up to a specially named binary rifle, in a cave on this same ledge is a mantis and a data pad with intel on it.
Due to the multiple approaches you can take here, I'm going to give you some useful tips as opposed to a set walk-through (if you want to do exactly as I did, copy the video). One of the hardest cores is the one blocked by a bubble shield inside a cave. Inside is a pair of hunters as well as several elites on ledges inside. You need to be extremely wary of the hunter's plasma blasts as they can one shot you and/ or any vehicle you are riding on.
This is where you're going to want to use that plasma caster I pointed out just before to take down at least one of the hunters. After that it's a case of wearing down the second hunter while luring him forward to make sure you're out of range of the elites inside the cave. If you do manage to find the mantis suit, you can take out a fair number of the enemies in the area far easier. Keep in mind that you will also continually be attacked by a single randomly spawning ghost every 40 - 80 seconds or so. Turrets will be guarding at least two of the cores, so be on the alert when you approach them for the first time as they can deal significant damage to you and any vehicle you are using. This is the second hardest area in the game that doesn't involve the Warden, so be prepared to die quite a few times. After the final core is shut down and the enemies in the area are gone, Exuberant will open a nearby structure for you to head into.
Objective: Defend the monitor
You will now be inside a large oval room with ledges all around the outside that have powerful weapons inside. Welcome to probably the hardest Warden free fight in the game. Hope you're ready. On the left are fuel rod cannons and an energy swords. On the right are rocket launchers and sniper rifles. Enemy soldiers with splinter turrets and light rifles will spawn in at the back of the room. Note for future enemy waves that you'll want to have access to these dropped turrets quickly (especially against knights). So it might benefit you to make sure that any that drop on the lower level or center of the area (what I fondly dub the 'death zone') are carried by you back up to the raised ledges. Use your light rifle or the sniper rifles to bring these lesser enemies down (conserve the more powerful weapons for later in the fight). You won't have much of a chance to use sniper rifles later as you'll mostly be running. You can also break off the turrets in the lower area so that spawning soldiers can't use them against you, increasing your weapon stockpile on the upper levels at the same time. The man cannons on the upper platforms should be your panic button if you are damaged and need to get away quickly, so keep this in mind.
After the initial wave of soldiers is mopped up, a group of additional splinter turret soldiers will appear in the middle of the lower area. Fire down at them and repeat the process you used against the earlier soldier wave. Exuberant will ask for your help in activating two console on opposite sides of the room a few seconds later (these are on the upper ledges). Use the man cannons to travel quickly from one side to the other. Annnd here we go. A swarm of crawlers will spawn in at multiple locations of the room. These bastards will hunt you down relentlessly, often destroying your ally AI in just a few seconds. While crawlers alone aren't much of a threat, they will leap up onto the ledges, often blindsiding you and just appearing in such numbers that it is difficult to stay alive. Keep in mind that if your team goes down on legendary, you've got no one to revive you for at least 30 seconds. If this does happen, you'll want to sprint in a loop around the ledges as hackety sax plays in the background, kiting them around the room until your allies revive automatically. Oh, and soldiers will also be running around attacking you at the same time alongside watchers.
While this wave isn't insurmountable, it's a taster for what is to come. From this point on, start using the dropped splinter turrets to bring down enemies faster. Another wave of soldiers, crawlers and watchers will follow this one. Then we get to start on the hard stuff. A trio of knights will spawn in with more crawlers. Now it's time to use your big guns. The fuel rod cannons and rocket launchers (although you will want to aim at the floor beside the knights with the rocket launchers as they can deflect the projectiles) as well as any remaining splinter turrets should be used at this point. Just make sure to bring down any remaining watchers first as otherwise your efforts against the knights will be pointless. Knights with splinter turrets will appear in the center as the final wave. If you have any powerful weapons left, turn them on them and use the AoE effects to tear them apart. When the last knight falls, you'll be prompted to get on an elevator in the middle of the room. Drop your turret if you're holding one as you'll want to be fast in the area below. Entering the elevator will take you to the core.
When the elevator stops, you'll need to sprint and dash as best you can along the linear walkway, triggering a scene when you approach the core. Watch the scenes play out and the ending credits will start to roll. Congratulations! You've finished off the fifth numbered entry to the Halo series. However, there's still plenty of fun to be had in the online multiplayer and co-op.