Halo 4

General information:

This achievement can only be earned in Spartan Ops, Episode 2, Chapter 3, "Hacksaw (Fortress)" on Heroic or Legendary difficulty. In this Mission you have to save six Marines - three groups of two - and make sure that at least one Marine is alive at the end of the mission to get the achievement. All Marines are invincible until you have saved the last group by eliminating all of of the initial enemies.

Short version:

Use a Ghost to push a Marine all the way down the canyon at the starting point of the mission while he is still invincible (before rescuing the final group of Marines).

Long version:

I did this solo on Legendary and took three different precautions to make sure at least one of my Marines survives in the end. Of the initial four Marines you save I placed three in locations from where they couldn't run around and do stupid things - like dying. ^^

IMPORTANT NOTE: I did this while they were still invincible - meaning: before rescuing the last two Marines.

1. I meleed the first Marine into a gap between the rocks and the weapon/ammo crates at the start of the mission. He got stuck there, but died in the end.

2. Also at the start of the mission I used a ghost to push the second one over the edge. He was trapped behind the little ledge you see, when you turn around at the start. He also died in the end.

3. For the third one I also used the Ghost to push him all the way down over the edge at the beginning of the mission. I killed myself in the process and destroyed the ghost, but he was still alive and kickin' at the bottom of the canyon. He survived.

All three survived the initial attacks of the Covenant until you have to move up the tower and the two Phantoms appear. Marines 1 & 2 were then apparently killed by the Phantoms (Just assuming this, since I didn't see it happening). The only one surviving was the one at the bottom of the canyon, because either the Phantoms didn't notice him down there or he was out of their range. Then I just went on and killed the Phantoms, the Wraith and the remaining enemies and plong: Achievement unlocked.

Hope, this helps you guys out. Please let me know, if you have any suggestions on the guide. =)

Credits for the video goes to teamextralife on Youtube (If the video helped you out, leave them a Like). :)
And also a big thanks to rafa1000house for finding the video in the first place and posting it in the comments. :)

The way I did this and I think it's probably the easiest way to do it is in co op on Heroic, and have 1 person stay with the marines at all times, because they like to wander around and get killed. Have that 1 person take them to the spawn and wait there the whole time while the other(s) kill everyone, when everyone's dead run to the top. try to have more than 1 still left at this point because a few elites come. Then when the phantoms come, just keep doing the same thing, 1 person staying with the marine(s), the other(s) destroy the phantoms.

I got this alone by signing in a guest account on a second controller. Just leave the guest player at the spawn point and go kill everything. Basically there are three phases:

1. Saving the marines

First, you have to save three groups of two marines. During this phase, the marines are invincible. Make sure you kill the two Hunters before you save all the marines. Then, save the two groups closest to the spawn point and let them retreat to the spawning point. They will stay there together with your guest player.

While the final group is still fighting off the Covenant, go into the tower to collect some cool weapons (rocket launchers and spartan lasers) and give them to the marines that are waiting at the spawning point. The three buildings where the marines are hiding out, also contain a rocket launcher each. Go back into the tower and get another rocket launcher and spartan laser and hold on to these.

In the outer areas of the map, there are two ghosts you can pick up. Bring one back to the spawning point and go get the second one to kill enemies (splattering works great against Elites).

Finally, save the final two marines and get them to follow you back to the spawning point immediately. Give the rocket launcher and spartan laser to the two marines you just saved. Grab the rocket launcher in the cabinet near the spawning point for yourself, just in case.

2. Defending the marines

Once you have saved all the marines, Covenant dropships will come with many Elites, some smaller enemies and a Wraith. In this phase, splatter or shoot everything that comes close to the spawning point. Don't worry about the Wraith yet, as it's not in range to be a threat.

I waited in my ghost on the right hand side of the spawning point, hugging the outer wall of the structure where two marines were being attacked, as almost all enemies come from that side. From this point you can look down the alley that takes you to the middle area. Lots of Elites will come through there. Some others will go around the back of the building, so make sure you check that alley also every now and then. Keep an eye on your radar.

If you die, go back to this place asap and try not to let the Elites get near the marines. As they have quite some firepower though, they can defend themselves somewhat. But better safe than sorry.

Once you cleared out all the Elites, Jackals and Grunts, go into the tower and destroy the Wraith with a spartan laser or rocket launcher. Then, immediately get back to the spawning point.

3. Destroy the Covenant ships

In the final phase, two ships show up which you have to destroy. Concentrate on all the Covenant on the ground first. Once you have cleared the ground, go into the tower and destroy the ships with spartan lasers or rocket launchers. Not sure about the rocket launchers, but at least the spartan lasers will have unlimited spawns in this phase. The ships will fire sometimes, but the shots didn't kill any of my marines.

Good luck, and ask if you have questions.

Here is a video solution that fully illustrates what El Cappitan was describing. Credit to him for the method!

Step 1:
Hijack covenant ghost

Step 2:
Use ghost to push a marine into the canyon by the spawn location

Step 3: Complete Mission

Step 4: Profit!

Last solutions are great for co-op, but this solution is for people who want to try this achievement solo, I did it solo on Heroic, and almost got it on Legendary, so I figured I would post some things that made it easier for me. First of all, until you have rescued all 3 sets of marines, and while they are still defending, or if 2 or 4 are following you, as far as I can tell they are invincible, I was trying this first on Legendary and they were taking direct hits from fuel rod guns, getting meleed non-stop by elites, and smacked around by hunters, to no avail. Perhaps this is solo only, I am not sure. Somebody let me know if I am wrong about this. Once you have rescued all 3 groups of marines, more covies will drop in. The most dangerous are elites with swords and cloaking, kill them quickly. The method I went about this is to save 2 of the groups of marines, and then climb to the top of the structure in the middle, there is an entrance on the ground, which leads to a hallway with an elite on a plasma turret, kill him with a shotgun and don't destroy the turret. Once you are at the top of the tower with 4 marines you will find lots of rocket launchers and spartan lasers, I gave 1 of my marines a launcher, the rest had shotguns or battle rifles/DMRs. From the top of the tower kill the last group of covies, and the marines on the ground may or may not make it up to you. Here is where it gets tricky. The first wave of covies after this did not come up to me, but killed the 2 marines that had stayed on the ground. Kill some of them until you are warned more are coming, and then jump on the plasma turret, and kill any that come up towards you. Make sure to have a shotgun and rocket launcher, or you will be overrun here. Once they stop coming, go back up to the top and kill more from here, until the 3rd wave comes. Rinse and repeat last step, then the 2 phantoms will come in that you have to kill. Take them out with rockets/lasers and clean up the stragglers, and bam, achievement unlocked. I had 2 marines left solo on Heroic, would have had all 4, but one was knocked off by a phantom concussion shot, and died from fall damage, other was killed by a cloaked elite with a sword. If anybody has more useful suggestions or ideas feel free to send them to me and I will edit solution. If you downvote solution please tell me why so I can improve it. Hope this helps!

This achievement can be obtained on chapter 3 Episode 2 'Hacksaw, Fortress' make sure you are playing on Heroic as obviously this is easier than legendary.

I recommend you do this achievement co-op as it can be difficult, you have to save groups of marines whilst fighting off the covenant.

When i did this achievement with a friend all of the marines got killed except for one who got glitched/stuck in a wall and no covenant would attack him, if this happens keep progressing with the mission as he will count as a saved marine.

If this doesn't happen then i suggest try running to the groups of covenant before your marines do to defeat them before your marines get killed, also try to hijack the ghosts and destroy the wraiths straight away.

At the end of the mission you have to destroy two phantoms, pick up the spartan laser to take these out as quickly as possible.