You start the level in a corner of the map. The area consists of 3 structures, each with a room inside and and a large, multi-floored structure in the center. This entire area is identical to the section of The Campaign where Master Chief disables the first pylon in the "Forerunner" level, except everything is covered in snow.
Your first enemies are a small group (about 6) of Crawlers near the first objective point. Due to the more open terrain there, it is best to hang back and snipe the Crawlers from afar, because rushing to the area will allow the Crawlers to swarm you in a group. DO NOT approach the objective until at least you kill 4 of them. One of the Crawlers may also have a Bianary Rifle, however, so watch for a Crawler with a red glowing circle around it. (don't take the weapon, however, because it won't be very useful for this level anyway.) The main weapons you will be using are the Light Rifle, and either the Boltshot, Scattershot, or Surpressor to complement it.
Once you reach the the jammer by dropping down the slope to reach it, another 6 Crawlers spawn, this time supported by 2 Watchers. Because of the close proximity, it's best to dash around the the tower to the left of the objective as soon as you see the dots on your motion tracker. You can catch some of the Crawlers just spawning, and can deal with them before the Watchers can get close. Use a Magnum, Boltshot, or DMR/Battle Rifle to head shot the Crawlers at first, then if they close in to try to swarm you, you can switch to an Assualt Rifle or Surpressor and finish them. Because the Watchers have to move in close to support the Crawlers, they can be killed with 5 shots of a ranged weapon on a burst from a close quarter one.
When the enemies are eliminated, don't go for the next objective just yet. Head up the ramp of the closest small building and you will find a Promethian ammo crate, with 2 Boltshots, a Light Rifle, a Surpressor, and a Scattershot. Take the Light Rifle and Boltshot, and then exit the "boxed" area of the walls on the far side, so you are closer to the main/center building. Approach the "activate" marker on your right.
As you approach the next of the small buildings, another group of Promethians will ambush you.3 Watchers appear out of a portal in the sky above the building, and 10 Crawlers spawn on the ground around the boxed area. Quickly move back and shoot the Watchers as they move slowly out of the portal at first, allowing you to hit one or 2 of them before they move away. Then let the enemies spread themselves out trying to reach you. If the Crawlers come first, they tend to rush closer to use thier Surpressors. Hit them in the head with a Boltshot, and if the Watchers see you first, use the Light Rifle to deal with them. Each Watcher dies in 5 shots (half the magazine) of the Light Rifle, but they if they take 3 shots they start to fly away, so you'll have to track them carefully. When the first 3 Watchers die, another 3 appear out of a portal in the same spot to take thier place. Make sure all Watchers are dealt with so they don't start setting up turrents. Also, 2 of the Crawlers will hang back near the switch because they have Bianary Rifles. Take them out, but if you need any more ammo, grab it BEFORE hitting the switch. (There is also another Light Rifle between 2 small boxes in the corner of the walled area, and a rack of Scattershots on the wall closest to your approach.)
When this is done, you should turn around and head back to the first building (the one closest to your original spawning point) and get ready for more foes. 3 Watchers come out of a portal above you, and you'll have to take them out before the join the main group of Promethians on the far building that you haven't reached yet. Refill your Light Rifle and take out the 2 sets of 3 Watchers that will appear over the last building, and the 10 or so Crawlers joining them. A turrent or two may have spawned as well at the top of some of the large platforms nearby. Once all foes are eliminated, stock up on more ammo and hit the switch.
Quickly enter the large building and prepare for another fight. 3 Crawlers spawn initially at the top of the slope entering the building, so wait for them to enter inside and approach you to eliminate them. If you stay inside, the rest of the wave (6 or so Crawlers and 3 Watchers) will be forced to funnel inside the building and you can eliminate the Crawlers without the Watchers being able to support them or fire at you from above. Emerge from your cover after most of the Crawlers are gone (one has a Bianary Rifle and will probably stay outside) and kill the rest of the enemies. When you get close to the switch, another set of Watchers (guess how many) will try to ambush you directly overhead, but if you move back quick enough they shouldn't be a problem. Hit the switch, and kill the 5-6 Crawlers that appear on the ground far below. (Again, one has a Bianary Rifle. Be careful, on Heroic one shot can weaken you enough so that only a few passing boltshots from a another foe is all it takes to kill you.)
You can rush to the objective, but the Promethians will try to ambush you one last time when you get there. Another set of Watchers and 3 or so of Crawlers immediately set on you. On higher difficulties, some or all of the Watchers will immediately fly down and spawn another set of 3 or 4 Crawlers/one turrent each, potentially bringing the number to up to 15 Crawlers, all in one area and with no cover! It's easy to get killed quickly like this. But there is a way to escape the trap. When you get about 20M or so of the landing zone, the music changes, signalling combat. IMMEDIATELY turn and run back to cover, so when the 3 Watchers spawn they'll be in your sights. The Crawlers will be forced to run at you on open ground, so you can pick them off fairly easy. When the first set of Watchers are gone, a second set will appear. Unfortunately, because of the distance if you choose to run (or distraction from the other foes if you choose to stand your ground), the other 3 Watchers will spawn thier Crawlers, and so you'll have to fight a second set. Stay in cover, use the Boltshot on the Crawlers and Light Rifle on the Watchers. Try to kill the Watchers from a distance first before you have to fight the Crawlers up close with the Watchers supporting them. With any luck, you'll make it without dying and the fight will be over.
A few more tips/reminders:
1.The Watchers will always run after a few shots. You have to track them one at a time to eliminate them. If you're using a Light Rilfe, it takes 5 shots. Don't use a Boltshot, since they can quickly move out of the way of those due to the short range, and if they dodge just one, you'll have to reload and you've wasted a whole clip.
2.The Boltshot is much better against the Crawlers than a burst from the Surpressor, because the Watchers won't have time to put a shield around them.
3.Because there are no Covenant to be fought on this level, there are no Covenant weapons either. There are also no UNSC ammo crates as well, so it's a good idea to use a loadout with Promethian weapons.
4. The Promethians tend to fight by swarming with the Crawlers numbers and the Watchers providing shields and spawning turrents. If you put more distance between you, only some the Crawlers and Watchers (the ones closest to you) will try to chase you, so you can force them to spread out and eliminate them one at a time.