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!