The mission starts when you are deposited on the right side of the end of the area known as "Sniper Alley". Unlike the level of the campaign "Reclaimer" where you must fight your way up to this area, you are going the opposite direction.
On the upper deck you start out on, a pair of Grunts will immediately come into view. Deal with them, and then turn to your left and move to the UNSC ammo crates there. You will find a Battle rifle that will be useful if you don't already have a long range weapon.
Return to the area you started out in and head down the walkway where the first two grunts came from. Another ammo pile with assault rifles are there. At this point, you should come under fire from the Covenant squat below you and the Shade turrets flanking them.
The Shades are first targets you should take out, especially in higher difficulties when they can target you very quickly and effectively. The first shade is across from you a good distance away on a rocky out cropping. Take cover behind the wall of the deck and fire your ranged weapon until the elite manning the turrets is dead or the turrets itself is destroyed.
Next, move left across the deck so you are facing the next turret. Take it out as well, and the squad below can be dealt with. The squat consists of three Elites, about six Grunts, and three Jackals. When there are five or so covenant left, they will be marked. Beware, the highest ranking Elite carries a Concussion Rifle, so stay up on the deck to shoot him from a safe distance. Disable the shield barrier as per instructions, and prepare for the next set of foes.
(If your preferred range weapon is the DMR, note that there is a third UNSC ammo pile with one in front of the rock platform of the right Shade turret.)
When the shield disappears, move to cover immediately. A group of three Jackal snipers will be right across from you on the second raised platform ahead and will fire at you with their carbines. Below them, a group of four shielded Jackals will be reinforced by a drop pod with several Grunts and an Elite. About three of the Grunts are carrying Fuel Rod Cannons, and on higher difficulties will shoot very quickly. Since this group is closer to you, more will be shooting you at once. Deal with the Snipers first, then get the Fuel Rod Grunts and their Elite commander. Mop the rest of the foes last.
When the switches to take the down the second shield appear, activate the bottom switch BEFORE turning on the second one one the platform. When you move in front of the second switch, turn around 180 degrees and you will see a pair of Beam Rifles hidden in the shadows of a sort of metal shelf. There is also a Covenant weapons crate with standard Covenant weapons nearby.
Take cover just like before when you hit the button, as a Phantom will fly into view and shell your area. It will deposit 2 snipers on the bridge below you before leaving. Take them out, and a drop pod will release another squad further back in the rocky area to the right. It drops six Grunts and two Elites. Some of the Grunts are carrying Fuel Rods, just like before. Since the Elites are carrying Storm Rifles, they will charge your position and try to get in close. Kill them first, then take out the Fuel Rod Grunts and then the remaining Grunts and shielded Jackals.
When this is done, more drop pods will appear, deploying three Elite Rangers and a Zealot carrying a sword. If you still have enough ammo for the Beam Rifles, you can use that to snipe them. (It's recommended to only use the Beam Rifle for high value targets like Snipers and Elites, and to use something like a DMR for Jackals and Grunts.) If you ran out and moved down to the bridge, you can use the Needlers on the nearby crate to kill them quickly. The Zealot carrying the sword generally comes last, but watch for him because he can kill you in just one hit on higher difficulties.
A phantom will then appear, dropping more Covenant further back as you move away from the metal base and into the winding-path rocky area. The Phantom drops about four Grunts, two Elites with Storm Rifles, and two Jackals. Stand your ground near the ammo crates on the far side of the bridge and use the needlers if they get to close.
A drop pod sends another squad at the very back of the rocky pit, depositing four Jackal Snipers all bunched together, a pair of Shield Jackals and 4 Grunts. Kill them, but stay alert for one last drop pod carrying an single Elite with an Energy Sword. Keep the Needler in handy to deal with him quickly.
When you enter the final combat area, Command will drop you some power weapons (a Railgun, a Sniper Rifle, and Rocket Launcher). Since the Rail gun is very close to you, swap your depleted weapon (assuming the Beam Rifle has been all used up ) for it. But WAIT, don't go out to grab the other ordinance yet!
This last set of enemies can be tricky. A Shade Turret (with some Grunts to man it) and two Snipers are on the far end of the canyon, and will pin you down at the quarry exit. About four Jackals and 8 Grunts will be directly in front of you, so you'll have to alternate between dealing with the snipers, turret, and little guys all at once. Use your DMR/Battle Rifle to take out the Snipers and Covies, and use the rail gun to target the Shade Turret itself. A Phantom is also dropping some more troops to the right on the other side of some rocks. The troops it's carrying won't appear until a little later, but take cover from the Phantom's guns and wait for it to leave before resuming the fight.
A drop pod appears later, depositing more foes. It's a tough crowd, with at least three of the six Elites are Sword bearing Zealots all trying to charge you. If they get too close and you have no short range weapons like an Assault rifle, Storm Rifle, or Needler, use the Rail Gun by charging it quick and aiming straight at them. Follow up with any grenades you have left.
After all Elites immediately fighting you are gone, take a few steps forward and look to the right. There's a floating platform with two more snipers on it, so take them out before rushing to the left to collect the power weapons.
After the snipers are gone, they're should only a be a few marked stragglers to take care of, and once you're done with them, all that's left to do is to mark the generator. MAKE SURE to move past the area of the floating tower platform, so you don't get caught in the blast radius of the artillery strike!
A few tips for playing on Heroic or harder:
1.It takes about a full clip of Assault rifle bullets to take down an Elite, and about 6 or so shots from a DMR. They tend to be very evasive when they're being sniped, so stay frosty. If you've got a plasma pistol handy, and overcharged shot can hold the Elite in place long enough to snipe his head.
2.The Phantoms are very accurate when they fire at this difficulty, and tend to fire in several salvos. Stay in cover and don't emerge until they leave, as combined fire from the turrets, chin gun, and any snipers or lucky covenant is more than enough to bring you down if you aren't careful.
3.Since there are no Prometheans on this level, there are not Promethean weapons/ammo.