From Halopedia, the Halo wiki
Torch And Burn (Mission Start)
The start of the level puts back at Farragut Station, but not as you remember it from ONI: Sword Base. The area has flooded and there's tight Covenant defenses. Looking out from the ridge, you'll see two Shade turrets, a Ghost driven by a Grunt, three Heavy Grunts with Fuel Rod Guns, a lookout tower with a Jackal Sniper, and many more Grunts behind what was the comms building. You have three Orbital Drop Shock Troopers as teammates, but they won't last long against all those turrets or Ghost. As for yourself, you have enough DMR ammo, a Hologram armor ability in the building in front, and the high ground.
Your first plan is to stay back and pick off everything you can from up here. As soon as you drop down from the cliff you're on, the Covenant will be alerted and your teammates will rush ahead and get themselves killed. The cliff is pretty narrow and invisible walls prevent you from climbing higher, but you still have a decent amount of room to shoot at the Shade turrets and the Jackal sniper. It's also harder for them to shoot you at this distance, though still easy enough if you stay put while they shoot. Also look for the Fuel Rod Grunts. You'll be able to headshot two of them for now; the third is behind the generator building and can't be seen yet. Also see if you can kill the Ghost's driver. This is much harder because it doesn't stop moving and because the Grunt driving it has his head protected by its cockpit. So aim carefully with bodyshots, and you might possibly take him out. If you can't from here, move up into the comms building and use the Hologram there to distract it.
Once the Ghost is dead, move up. There's still a lot of Grunts left, including one more Fuel Rod Grunt. Your teammates will cross the lake and attack the Grunts from the right, so you can either join them or come from the left as a flanking maneuver. Sometimes they'll even drive the Ghost, which can be pretty helpful. When you hear Emile and Jun start chatting over the comm, that means everything's dead. Hand Fuel Rod Guns to your teammates before they run off to the next area, grab a plasma pistol for yourself, and restock on DMR ammo before heading off with them.
A Spare Tank
Unlike the tank sections in Halo 3, the Scorpion should be used with caution than with reckless abandon. It responds to enemy fire more, having weaker armor and a screen heating effect when it's shot at. It's more powerful than in Halo 3 but has a slightly slower firing rate, and your teammates, even armed with Fuel Rod Guns, don't have much effective range. The key word of this area is vigilance. Stay alert of everything coming and shoot before you can get shot at.
Anyway, first headshot the Grunt gunner on the lookout tower overlooking the Scorpion. Then jump in and slowly go around the boulder it's parked at. On the other side is a Shade turret, past it is another, and then past them is a bunch of Elite and Grunt infantry. Coming down the mountain after them will be two Ghosts. Try to engage everything one at a time and watch your reticle for when it turns red. Corners and boulders are ideal cover, and if you see an glowing blue explosive plasma batteries, shoot them to mop up infantry with them. Once they're all destroyed, drive up the mountain to where Airview Base used to be.
The Tyrant Guns
As you come up the hill, you'll see two Tyrant anti-air guns. As they're immobile, huge, and scripted to shoot at the UNSC aircraft on the other side, they're no threat to you. It takes eight shots to destroy each with the tank. The bigger danger is Banshees. There are roughly eight to ten Banshees flying in this area, and their weapons are powerful. Should they gang up you, your tank is not likely to make it out. So you have to progress through this area slowly and cautiously, watching for any Banshees that are flying by. Shoot them down with the tank; it only takes one shot to destroy each. Also, your tank shell is not hitscan, so you'll have to lead your shots a bit.
When both Tyrant guns are destroyed, several Falcons, one of which is carrying NOBLE Team, will fly down to help you out. You will be allowed to progress through the area. Keep an eye out for infantry and use the explosive canisters to kill them. There's also a Ghost or two in this area, though the Falcons sometimes take it out. Stay alert, and head on to the gate.
Fuel Rod Turrets at the Gate
Now comes what is perhaps one of the hardest sections in the level, if not the game. In what was the western plains, you'll have two Ghosts and a lookout turret with a fuel rod Grunt in it. Past them are four Shade turrets and two Revenants guarding the gate. Two of the turrets fire rapid-fire plasma shots and the other two fire fuel rods. Not only do these fuel rods travel faster than you'd think, but the turrets are so far away that you have to know beforehand where they are to shoot them. And on top of that, there are no checkpoints in this section until the whole area is cleared. Screw up one turret, and your tank explodes and you have to do the level over again.
So when entering, first aim for the lookout tower and blow up. Then rev forward and take out Ghosts as you see them. When you come to a large boulder bordering this plain from the gate area, carefully rev around and shoot to your right to blow up a fuel rod turret. If it fires, rev back immediately so its shots hit the boulder, then try again. When it's blown up, drive around the rock and blow up a Shade turret that'll be on your right. Ahead of you in the valley you'll see a Revenant, shoot it, and past it will be another far-off Shade turret.
As this point you should be against the left wall of SWORD Base in prep for the last and most difficult fuel rod turret. This one is on a mountain high above you, and is so close that the tank can't dodge its shots. So, use the base corner for cover. Position your tank so it's facing the wall, then carefully rev backwards and fire at the turret. If it fires back, drive forward so its shots miss or hit the wall. The tank is slow but you need fast reflexes for this; drive forward as soon as it shoots.
When it's blown up, the objective may say the area is clear, but it really isn't. There's one more Revenant hiding back right in front of the gate. Drive around the corner and keep moving so it doesn't shell you. Then once you see it, blow it up. The area will then be safe for you to park your tank at the open gate.
Latchkey (Rally Point Alpha)
The main door at the gate won't open, just a small parking spot, so you're forced to leave the tank and the ODSTs behind. But should you have a DMR and plasma pistol gained back in the first area, then you should be fine for this next encounter.
Head into the gate and up the road leading to the courtyard of SWORD Base. As you reach the top, you'll see NOBLE Team fighting two Elites, some Jackals, and a pack of Grunts. They'll be busy fighting your teammates with their backs turned to you, so you can easily surprise one Elite with a noob combo before finishing off the other. With the Elites down, Noble will mop up the Grunts fairly quickly and then head into the base. At its entrance ramp will be two Grunts manning plasma turrets. Approach them carefully to headshot them, and since they're immobile NOBLE might easily take them out themselves. Once they're dead the entire area should be clear.
The only complication of note with this area is that sometimes a BOB spawns in the balcony to your left. He'll have a concussion rifle and is as strong as a General. You can noob combo with caution, but keep your distance and don't try to fight him hand to hand. When you leave the area, you can bring a plasma pistol and one of the turrets, but neither will be that necessary. If you need a health pack, there will be one right by your teammates at the door to the elevator.
The next room will have a maintenance catwalk filled with nothing but Jackals and Skirmishers of various ranks. This part may look simple but it's actually deceptively tricky. The catwalk has no cover and huge sightlines for snipers forwards, backwards, and sideways. Should you rush out to run and gun, you'll get quickly mowed down as Jackals surround you and shoot at you from above. Once again, caution is key.
First, scope in and look carefully in the room to take out as many Covenant as you can. There will be marksmen with needle rifles far back and shield bearing ones close by, as well as Skirmishers that will rush up. Let them focus on your teammates and pick them off one by one. If you have the plasma turret, you can use it to wear down their shields, though be carefully about getting flooded with return fire. You can also use a plasma pistol overcharge to temporarily stun a shield Jackal as well, though it won't break their shield. You can also use the Hologram to lure out enemies. If you haven't got it already, you can get it from a case on the opposite side of the entrance room, though clear out enemies first or you'll get swarmed while trying to grab it.
When you can't see any more enemies, carefully exit the entrance to start going up the catwalk. Use your hologram to see if the path ahead is safe, and check all around you. There's two dangers when going up. First, Jackals that might be on the catwalk above and behind you, so check behind you often. Second, on Legendary there's a Jackal Sniper with a Focus Rifle who can quickly drain your shields and health. Use the hologram to lure him out for headshotting. There's crates full of DMR ammo on the ground floor and at the top, so don't worry about conserving at all. There's also an energy sword, but it's not very useful here against Jackals.
Keep proceeding to the top, picking off Jackals as you go. When you reached the stop and are sure everything in the room is dead, you'll need to rearm. Go back down and collect all the needle rifle ammo you can get; there will be more than enough to completely fill it. Then look for a focus rifle. You can get the one from the dead sniper, or you could grab a full one that's been left on a small platform in the room's central pillar. Go to the second floor and look in the middle to find it, then jump up to grab it. Leave this area with a full needle rifle, full focus rifle, and Hologram.
The Ruined Atrium
There are no enemies in SWORD Base's bombed out security station, but there's a lot of fully loaded weapons here. There's a full plasma pistol on the floor, which you'll take with you later, and full DMRs in a crate. Swap your needle rifle for one and then open the door to SWORD Base's ground floor.
There you'll fight numerous Spec Ops Elites, an Elite General, and an Engineer shielding them all. They'll take a while to enter first, so use the brief opening to take out the Engineer. Kill it with the focus rifle; there's no point in sparing it as NOBLE Team will just kill it anyway and it gives the Elites more shields every few seconds. The cloaked Elites will then rush in while the General hangs back for now. Use the focus rifle to wear down their shields then use the DMR to headshot them. You could possibly use the focus rifle to kill them too, but it'll expend ammo faster. You can also use the hologram to lure them out, as you'll take a lot of fire trying to move up in the room.
When all the Spec Ops Elites are dead, the General will come. He's got a concussion rifle and stands on top of a narrow ramp, making trying to engage him close-range with the noob combo far too risky. So use your focus rifle again to wear down his shields, and then a DMR headshot to kill him.
When everything's dead, go back for your needle rifle and leave the DMR. Then swap your near-depleted focus rifle for a fully charge plasma pistol you'll find near the entrance of the security room. Then go up with NOBLE Team into the first floor and push the button to initiate the cutscene.
This Cave Is Not A Natural Formation (Rally Point Bravo)
Dr. Halsey's lab needs defending from an entire of army of Covenant, and they're going to throw everything they've got at you. The only rank that doesn't show up is Brute Chieftains, but every other species does, as well as vehicles like Ghosts, Banshees, and Wraiths. But you've got more than enough armament at the holdout bunker to help you. There are multiple DMRs, grenade launchers, assault rifles, pistols, rockets, armor abilities, Mongooses, and a single Spartan Laser. And as the dialogue informs you, there are also four defense turrets that auto-fire at enemies and need to be reactivated when damaged. And of course, your Spartan teammates are there too.
It's important to note that playing on Heroic or Legendary will change the battle significantly, not just in the amount of waves but also in tactics. For instance, the turrets should mostly be ignored on Legendary. On the highest difficulty they will be at most just distractions, and are too far away to reactivate without getting swarmed on the way. Additionally, using a Banshee also changes. On Heroic, it's pretty safe to use a Banshee to rain fire down from above. But on Legendary enemy projectiles travel so fast and vehicle health is so weak that it's best to avoid fighting them lest you get shot down. Additionally, enemy Banshees will attack you, and a new one will spawn for each one you kill, so it's not worth it.
The safest spot in the bunker is right behind the sandbags. They can absorb huge amounts of fire, and you're basically invulnerable should you crouch there unless a grenade is tossed behind it, so stay alert and/or use Armor Lock. It's best to not go past the sandbags when fighting. Past there will be a lot of open land with no cover, and the enemies usually won't come past the sandbags unless they're berserking or kamikazing.
Your loudout at this point, as said earlier, should be a Hologram, a fully loaded needle rifle, and a fully charged plasma pistol. After activating the first turret, head to the bunker with your team. Your priority here right now is to hijack a Banshee before the enemies turn up. This Banshee will be for dealing with the Wraiths that will spawn every two waves. Deploy a hologram to lure and distract the two overhead and look for a Jetpack (and grenade launcher if you want to save plasma pistol ammo.) Fly up to the roof of the bunker and EMP a Banshee when it comes close, then board it at the prompt. When it's yours, fly it to either the left or right side of the bunker and park it there. You'll be stashing it here for now to keep it safe from enemies. Then grab a DMR and keep your needle rifle handy as you wait for the Covenant infantry to come.
The first wave is Jackals, Grunts, and Brutes. The minor infantry will come first, and you should be an expert in killing them by now. Most of them will be distracted by the turret, but it will quickly shut down under their fire. The Jackals will cost you the most DMR ammo, and though you have a lot it becomes scarce as time goes on, so it pays to conserve.
Brutes will come shortly behind them. All the Brutes are Captains, wielding plasma repeaters or spikers. Since their helmets are so durable on Legendary, use the needle rifle to explode them. Be careful with your ammo, as you won't get any more during this entire battle. The best time to shoot them is when they're about to go berserk or throwing a grenade, since they'll be stuck in an animation and unable to dodge or use Armor Lock. If they berserk, they'll come past the sandbags. Fall back so they focus on your teammates, and stick them with a plasma grenade.
When you hear Emile say: "Watch out for the Wraith!", go to your Banshee and fly up high. Flank and destroy any Banshees you see so they don't get you; one fuel rod and then tons of plasma shots should do it. Then target the Wraith with locked-on fuel rods. It'll take four to five shots to blow it up. Take care to not fly too low or its gunner will shoot you with its plasma turret. Then fly back to the bunker and park your Banshee again. Don't fall into the temptation of trying to blast the infantry with the Banshee; they will shoot you down. Mop up the rest with your DMR and needle rifle.
Don't listen to Jun. Anyway, the second wave will be Elites of various ranks, plus a few Skirmishers, Grunts, and Jackals. Collect a plasma pistol or use the one you brought from earlier; your needle rifle won't be helpful again until later. The Elites will hold position a dozen meters or so from your bunker, so they'll dodge EMP shots. Instead, use the normal shots to "pepper" their shields then follow up with a headshot. EMP shots should only be attempted if they're close enough that the reticle turns red. Frag grenades will damage shields but plasma grenade sticks will not kill them on Legendary.
Most of the Elites will be Majors and Rangers, the latter whom have flight mobility but very weak shields. The biggest threats will be Ultras with concussion rifles and the Ghosts that come behind them. These Ghosts will quickly tear up your Banshee if you're in the air, even at maximum height. Try for them last; you can use the rockets on the left side of the bunker. As for the Ultras, watch out for their grenades. Equipping Armor Lock can be useful here if you need to survive a sudden stick.
NOBLE Team is actually rather helpful here, as they can put heavy dents in the Elites' shields, especially Jun. If a Ranger flies into the bunker area, fall back so it focus on them before taking it out. Also keep a lookout for Skirmishers. They'll carry plasma pistols, and will come close enough that often times you can kill them and grab their weapon during a lull in the fight if your current plasma pistol is running low.
This wave is only on Legendary, and when you hear the percussion beats begin, that's when you've known it's started. It can start when there's a few Elites and the Ghost still alive, so keep an eye out and kill them quickly before the next Phantom arrives.
Soon a Phantom will drop off Hunters and some Brutes. But fortunately, you've got something set aside for the two behemoths: a Spartan Laser. Grab the laser laying in a case right in front of the closed door, then carefully aim at the two. It takes two laser shots to kill both, so don't waste any of the four shots. Hang back and crouch so the Hunters don't see you while you aim. If your teammates get in your way, it's best to stop charging and try again rather than risking a missed shot. If the Hunters survive, use any remaining rockets or plasma grenades.
When they're dead, take out any remaining Elites, Jackals, and Brutes. It pays to keep tabs on your needle rifle in the previous waves, as it might despawn if you don't pick it up every once in a while. There won't be nearly as many infantry here as there were the previous waves, so this part should quickly pass.
Those words and the new music mean the beginning of the final wave, which brings two Wraiths, fuel rod Grunts, an Engineer, and an Elite General with an energy sword. The Wraiths should be taken out first; blow up them and any remaining Ghost with your Banshee's fuel rods. One will be at the bridge where you and NOBLE entered, and the other will be towards the end by the very edge of the map where troops were dropped.
When you hear Halsey say: "Well done, Spartans. I'm opening the laboratory door", it means the level is almost complete. Kill any infantry left alive then land your Banshee and hit the door's button to end the mission.