From Halopedia, the Halo wiki
The vast majority of enemies in the level are Grunts. Each group of hostiles is led by at least one Brute. Jackals appear infrequently, with Carbine-wielding Jackals being quite rare. Weak weapons, such as the Plasma Pistol and Spiker, are very common. Power weapons are nearly non-existent, and scoped weapons are rare until late in the level. Vehicles, with the exception of non-drivable ones like Phantoms and Pelicans, do not appear.
New in Sierra 117
Walk It Off (Mission Start)
The start of the level is very linear; you should have no difficulty following the trail. When the Phantom flies overhead, you should see a high rock on the right, just the right height to be jumped onto. Go there; it will be an excellent perch in the battle that is about to ensue. Unfortunately, on Legendary, you have a pretty useless weapon set, so just use your pistol to fire upon the gold-armored Brute. As soon as the magazine is empty, get in motion. About five or six Grunts will show up from the left end of the log in pretty rapid succession. Headshot them—it should be easy, as they walk in a straight line—and finish off the Brute, assuming that the Marines haven't already taken care of him.
When that group is down, find a Plasma Pistol to dual-wield with your Magnum, and move forward until you hear the sound of another inbound Phantom. Back up and take cover behind the log, and be quick about it, or the Phantom's cannons will mow you down. As soon as the Phantom departs, move forward and take out the pack—the group of four or five Grunts led by a Brute. Take out the Brute first—it should be easy to Noob Combo him, as he is likely unaware of your presence. The Grunts will probably panic, but some may remain calm, so drop the Plasma Pistol and keep the Magnum ready. When the hostiles are all down, swap the pistol for a Brute Shot.
Alternatively, you can try to stick the Phantom's door gunner as it drops off reinforcements. If you succeed, you will easily kill three or four Grunts and secure yourself a Plasma Cannon for later use.
There are two packs farther up the trail. Both are led by Brutes with a Brute Shot; the skull's melee and the Noob Combo are both excellent ways to dispatch the first. The second likes to hide behind trees, so watch your back and check your corners. When these two packs are down, drop the skull, restock on Brute Shot shells, and keep moving.
The next encounter is not very far ahead. It takes the form of a large, non-linear basin. Inside are about fifteen Grunts (half of which are asleep), five Jackals with shields, three Carbine-wielding Sniper Jackals, and five or six Brutes. This isn't an easy battle, but it is manageable. Before heading in, swap out your Assault Rifle for a Carbine. Then, quickly move downhill from tree to tree, assassinating sleeping Grunts. Once you have finished that, wait for a small group of enemies to approach the group of Plasma Batteries on the far left side of the area. Carbine the batteries for a multi-kill.
The three Snipers, being the longest-range foes, are your priority. One is quite close, on a rock one level down from the place where you entered the area—right below the Arbiter. In fact, the Arbiter may already have killed him. The two others are harder to find; they're hidden in the foliage on the ridge up at the top right side of the area. Pinpoint them by their shots, and then zoom in and Carbine-spam—there is plenty of Carbine ammo in the area, so you need not worry about wasting it.
Once all three are dead, it's time to head down onto the valley floor. Fight your way across the river, taking care to look both ways for enemies as you cross. If you have picked up a Bubble Shield by now, deploy it on one of the raised areas just across the river to establish a beachhead. Target the Brutes first, then the Needler-wielding Grunts, then the Jackals, and then the regular Grunts. With the Brutes, you may want to use the Brute Shot's melee to knock off their armor before you go for the head shot. By the time you've cleared the far bank of the river, you should have reduced the enemy forces to a single pack. Swap out the Brute Shot for a Needler and use it to kill the last Brute. Mop up the stragglers with Carbine fire. Restock on ammo again, and move on down the path.
Up on the log above, Sergeant Reynolds is being interrogated by a Brute. Needle the Brute, and then move forward under and past the log to get up the slope. There's a Carbine Jackal and a Grunt or two here, so be sure to kill them before moving on. Past this area is a section off to the left, where a few Grunts wait for you to approach. Kill them, and move into the place they came from. Here, you can pick up the new, slightly better Battle Rifle, or keep your Carbine. At the far end of this area is a pair of Jackals (without Carbines, thankfully) and a pack. Again, use the Needler on the Brute and the Carbine on the Grunts. Follow the tunnel past the Cortana Moment and you've completed this section.
Charlie Foxtrot (Rally Point Alpha)
Immediately after you exit the tunnel, there is a small area off to the left, where a dead Marine lies with a Battle Rifle and some grenades. Take them as needed, and then follow the path until you find yourself overlooking a large area by the river. To your right is a corpse with a large amount of frag grenades; lob them all into the Covenant forces below. When you've used them all up, jump down the ledge, and move to the left; you'll find another frag cache here.
There are two packs in the valley when you arrive, but it's hard to say how many will retreat to the dam when the Phantom arrives. Usually, the Brutes fall back, but they are not your main concern. Across the stream are two Sniper Jackals armed with Beam Rifles—one-hit-kill weapons. Use extreme caution and terminate them from a distance. Once they're dead and the Phantom has left, you're free to begin the second and most difficult phase of this battle: attacking the dam.
This part can be obscenely difficult on Legendary, not simply because of numbers (though you will be vastly outnumbered) nor because of firepower (although you will be vastly outgunned) but because you tend to get nasty checkpoints. There are two ways to get up onto the dam: just jumping straight up from below (nearly suicidal but not impossible) or walking up the left path (slower but safer). If you want to jump right up, then go ahead and do it. You should move a lot, throw grenades, drop Bubble Shields, and watch your back obsessively. If you instead choose the left path, then beware of the four Jackals (two normal and two wielding Carbines) on the path itself, as well as the lurking Carbine Jackal up on a ledge to the left.
After the engagement at the dam, you will have to walk through the infamous "sniper alley". You should have a Bubble Shield at this point; deploy it if you find yourself under heavy fire. Once you've made it past the group of Sniper Jackals, you will find Johnson and a few Marines retreating from the Covenant infantry that have gathered around their crashed Pelican. Make your way through here, taking care to avoid the numerous Sniper Jackals perched on high ledges and tree branches, and make sure to take the Battle and Sniper Rifles from the Pelican's wreckage.
Quid Pro Quo (Rally Point Bravo)
This next part happens fast, so be ready as soon as the cutscene ends.
There are two methods here, and both will do job. The first one takes longer time, but is much more safer, as it will protect you from the Jackals. The second way is quicker, but you will be exposed to Brutes and Jackals.
After the cutscene ends, use your Sniper Rifle to take out the Beam Rifle Jackals. There's two of them, one walking on above the prison and one on the buildings behind the prison. Kill the one standing on the prison first, because the other Jackal will probaly not notice where the shot came from. Snipe him too, then swap to your Battle Rifle. Kill the Jackal with a Carbine just below you. Poke your head out, kill the alien that back pedal to safety. Now, you will notice that the Chieftain is on the dam. Four shots will kill, then his Brutes will roar and charge. Kill them as well and your Sniper Rifle. Take care of the Ultras first, as they posses stronger weapons and shields. If you missed some, wait until they come through the dam, then follow Arbiter. Kill the Grunts, and when you finished, the Arbiter should have finished the Brute stragglers with his Sword. If he dies, hide behind a rock or a box until the Arbiter rise.
Once you reach the dam, there will be two shielded Jackals and two Sniper Jackals on the ridge. Kill them with the Battle Rifle. One Brute will always be there behind beside the Jackal with his Grunts, and he will always deploy a Bubble Shield and take the defensive. Kill the Grints with the Battle Rifle, then quickly charge to the Jackal's corpse then hide behind a box or the ridge. The Arbiter will send the Brute on his Greatest Journey.
Before you've turned around the corner, swap you sniper for a Carbine. You will need it later. Turn around the corner then kill the Grunt calling for a Cabine headshot. There usually will be no enemies between you and Johnson. Take the Chieftain's Hammer if you prefer close combat. Save the marines, then wait patiently behind a supply cache. Headshot the Grunts coming, then wait for Hocus to kill the Brute Captain leading the Grunts. Jump on the Pelican than pat yourself on the back - you've just finished the first mission of Halo 3 on legendary.
Look to the dam straight ahead of you, but step back a little. There are two Jackals with Beam Rifles across the way, and several armed with Carbines scattered around. Zoom in with your sniper rifle, and look at the right side, near the building on the right end of the dam. There should be a Brute Captain Major standing there. Take him out with one or two well-placed sniper rounds. Next—remembering that all of this happens quickly—look at the left end of the dam (as far left as you can see, where the building on the left end of the dam starts). There should be a Brute Captain Ultra standing there. Take him out the same way you did the major. Next, another Brute Captain Ultra will run past the one you just killed. Occasionally he will run past the one you are firing upon, giving you two kills for one shot (which is nice when you have no more than 24 rounds to work with). Reload now, even if you still have a round in the rifle. Once the two Brute Captain Ultras are dead, a Brute Chieftain will run out from the same area the second Brute Captain Ultra ran out from.
The Chieftain only has Invincibility activated about ten percent of the time, and it is possible to take him out as he runs across the dam. He will sometimes stop if you hit him a few times, just shy out of cover, and end up being an easy target, but since he is sprinting, your first shot will have to be a good one to stop him in his tracks. Either way, try to kill him before he activates Invincibility—it is a useful powerup to have while being attacked. Three to four sniper rounds are enough to take him down if you land all of them on his head or shoulders. It is critical to land a first good shot on his head or he will most likely escape you. If you do happen to miss, don't despair. Look at the building on the right end of the dam where he disappeared into. About one third of the time, he goes downstairs and emerges from the door at the base of the building on the right side of the dam. Here is your opportunity to finish him off, all without having to step out of cover. He may also move down to the building in front of you and just stand there. If he does, leave him until after the snipers are gone.
After you take him out, jump into the air and take a quick scan of the area below you. Nearly all the time, there will be another Brute Captain Ultra standing in the open. After you jump line your sight up on the ground in front of you, so when you jump, you barely have to aim at the Brute below. One shot usually just knocks his helmet off, but you can jump again immediately and finish him off with a second shot to the head.
Now switch to your Battle Rifle and plug the Sniper Jackal down in front of you. Switch back to your sniper, stay down, and wait until you can draw a bead on the rest of the snipers (there are about five of them).
Upon rescuing Johnson from the makeshift prison the Brutes made, you will have one final engagement. Two Phantoms will arrive dropping a number of Brutes and Grunts. Engage them on the bridge and kill all of them. Alternatively, you can stay behind the prison, using a nearby box as cover and scoring headshots on the first wave of Grunts. Pelican Kilo 023 will then fire upon the Phantoms, destroying them easily. Hop aboard the Pelican to complete the level and begin the cinematic.
The Iron skull can be obtained in this area with minimal work.