This level basically consists of 2 parts: Attacking the Covenant stronghold, and then defending the subsequent Promethian counterattack. On Heroic or harder, this is an EXTREMELY difficult level, especially on the latter half.
You start at the very back end of the level, at the bottom of a small incline. Proceed just a few steps up the hill, and you'll see a pair of jackals(2/10) on each of rocky pillars dividing the approach into 3 seperate paths. Hang back and snipe them, then turn your attention to the numerous Grunts at ground level. (There's about 40 Grunts in the entire level.)
about 4 Grunts coming in pairs will march toward you down the left, right, and middle paths. Don't enter any of the paths yet, just wait for the Grunts to approach and you can pick them off easy. In the middle path, and Elite is standing in plain sight with a Carbine. Take him out, but be careful about getting too close to the middle path, because one or both of the Shade turrents can hit you easily from here.
Now you are going to start probing the left and right paths. This means you start to enter one of them, and wait for enemies to come to you. At this point, multiple Grunts and Jackals coming in groups of 2 or 3 will appear one at a time on the 2 side paths. Take them out, but always watch for the Shade Turrents. The left path is a little easier at it offers more cover and the Shades are at a less favorable angle there.
No matter which path you take, 1 Elite is at the end of the path. Hit each one with the Covenant Carbine (because it's the only ranged weapon with ammo in the area) until they fall. Stay inside the paths, and don't exit until everybody gaurding the approach is dead.
If the Shades are still firing, it's time to take them out too. Each turrent is manned by an Elite, but killing him will only cause the Grunts to man the turrent in their place. It's better to shoot the turrent itself, or if you have a good aim with a Plasma grenade, to stick the turrent (although one stick won't completely destroy it).
Now that this is done, the only enemies remaining are actually on/inside the Forerunner structure. Pick a path and slowly move up. When you approach, the enemies will file down the ramp (each ramp has someone mounting a plasma turrent) to attack in waves. 6 or so Grunts will attack you in the front, while at the same time another group of Grunts led by an Elite will be moving out the path on the other side (the one you didn't take) and try to trap you. Toss a grenade into the front Grunt pile, snipe the plasma gunner, then turn around and deal with the other Covies behind you. Be very careful to watch for the Grunts carrying Fuel Rod Cannons (about 1 in every 4 Grunts in this group) and take them out even before you hit the Elites. Be patient and take them all out before even setting foot on the metal. (Check both sides just to make sure.)
If they haven't already, 2 of the 4 Watchers will probably try to fly over and attack. They can shield thier Covenant allies, but can't revive them. Shoot them out of the sky when the other threats are gone. Because the Watchers only start to flee when they're fired on multiple times, it's perfectly feasible to hit them with a shot from a Fuel Rod Cannon when they get hover close.
After this, you can either head up the ramp to finally enter the structure to take out the rest of the enemies, or if you prefer a less combative approach, head back to one of the rocky pillars near the 3 paths and jump to the pointy rock at the end of one of them. Use a ranged weapon to take out a Covie or 2 and the Promethian turrent at the very back end. Because of the distance, it won't attack you from here, and since it's completely immobile you can even use a Fuel Rod Cannon shot if you can aim it well.
After you take out what you can from here, move up the ramps on either side and prepare for close range fighting. Expect at least a few Grunts, a Jackal or 2, and one Elite on the lower deck. Before you head up the ramp to the final deck, take a look inside the dark room underneath the upper deck and feel free to do a little weapon shopping. I'd advise stocking up on Carbine ammo, and maybe take a Needler, Concussion Rifle, or Beam Rifle if you ran out of Fuel Rod. The next fight is gonna be tough.
Heading up the ramp, there will be at least 3 Elites waiting for you. One's got a Sword, one has a Carbine, and one has a Fuel Rod Cannon. Try to lure the Sword Elite down the ramp so you can hit him with a grenade/Needler/Fuel Rod/enough shots with a Carbine. Then carefully peek over the top and try to hit the Elite with the Fuel Rod Cannon. It's kind of difficult since the Elite is a very quick shot with his cannon, but if you have one of your own it's possible to hit him before he can hit you. Take out the Carbine Elite, but don't rest yet! The Promethians still aren't done with you!
About 6 to 8 Watchers spawn behind you in a semi-circle immediately after you kill the last Elite. They can overwhlem you easily, since there's little cover from them up here. Sprint to anything you can and start firing right away. If you still have a Fuel Rod handy, you can blast them quick since they hover in so close. Attack the ones immediately shooting at you and ignore the ones fleeing, it'll take them a while to come back to you.
After they're done, you've got some buttons to press. As you approach each one, 6 Crawlers and 2 Watchers appear to stop you. Try to move back up to cover as you headshot the Crawlers with a Carbine and snipe the Watchers. Remember that Watchers can prolong the fight if you ignore them, and that the Promethian's strength in thier speed and numbers. Deal with each set individually, and don't try to run from them to activate the next button, because that just adds to thier total numbers and makes the fight harder.
After pressing both buttons, about 6-8 Crawlers and 4 Watchers will spawn at the top upper deck, so you'll have to breach the same tough position yet again. Try to lure the Crawlers and Watchers away from thier eleveated position to attack one at a time, then finish the 2 turrents the Watchers placed. About halfway through the fight, Command will send you some ordinance back out in the rocks. DON'T GO YET. Kill the rest of the foes first, because the ordinance is a bit of a trap.
On the one hand, if you try to grabe the SAM, Rocket Launcher, or Rail Gun, you've got plenty of powerful ordinance to deal witht the Knights. But trying to get all the weapons puts you deep inside the enemy group with no cover. Literally every Promethian will try to fire on you. If you think you can handle this, go ahead. But for an easier fight...
As you approach the nearest Rocket Launcher, the first Knight will spawn (a regular armed with a Surpressor. If you think you can tak him in one shot, go ahead, but as soon as you do so, turn and tun back up to the base, all the way to the lower deck where the Shades were. The other Knights will have spawned and probably deployed a few Watchers, who in turn will deploy some Crawlers in addition to the ones already there. But since you're up on a high vantage point with a ranged weapon, the fight is much easier. Take out the Crawlers and Watchers first (you can use a Needler on the Watchers, it's very effective) and then hit the Knights. From this distance, it might help to use a Plasma Pistol to crack the Knight's shields before you head/face shoot him.(It'll also keep them from teleporting, since it'll only take 2 shots instead of 7 or 8.) Make sure all the Watchers are dead before you tackle the Knights.
Once this is done, another wave of Promethians will give it one last shot. One of the Knights is a Battlewagon, and may try to teleport up to where you are. Use a needler for him, then move up to the upper deck and stay in cover in the back. The Watchers will reach you first, then the Crawlers. Take them out, and the Knights remaining on the bottom will be easy prey (since they have no ranged weapons to hit you with).
1. Stick with Covenant weapons because there is plenty of them in supply.
2. Use the plasma turrents if you can't safely get the ordinance.
3. Hang back and take it very slow and patiently during the first half of the level.