This will take a few tries at least, but can be done solo.
Primary- Favorite long-range rifle. Recommended- Battle Rifle
Secondary- Plasma Pistol
Armor Ability- Jetpack
Mission- Episode 2, Chapter 3, Hacksaw.
Difficulty- Heroic. Marines will stay alive just a bit longer, and enemies wont be so brutal.
First Stage- Regroup
In this section, it matters not how much you die and how quickly you go, because the marines have so much health that they are effectively invincible in this stage. The order to gathering these marines for maximum alive-ness is from left to right of where you spawn initially. Every spot has an initial encounter, followed by a drop pod encounter. Don't be afraid to use rockets or grenades to wipe them out, as the marines are invincible. Once enemies are clear, give the marines weapons that they can fight with. I would suggest the Battle Rifle for all, though shotguns for one or two cant hurt, and i wouldn't suggest rockets, becasue they wont shoot them very often and hit even less. From group one on the far left of spawn, go to group two. Kill all as before, rearm as before. Grab a Rocket Launcher from here and decimate the Ghost and Hunters between here and group three in the back corner, because they don't count as enemies you must kill for stages to move on. As well as them, kill the manned turret in the main building up the ramp before moving on. At group three, kill all, rearm all, and the stage is complete.
Second Stage- Don't Die
From here on out, the marines can and will die. If they have headshot capable weapons or shotgun for rampaging elites, most of them should lat to the end, though no guarantees. It's also critical to minimize your deaths here, because its a painfully long trip back from spawn to the building, Immediately after finishing stage one, you should run up into the building and grab a Spartan Laser. The marines will walk and follow you in, as a nice bonus. A Phantom will be approaching from the opposite side of spawn with a Wraith attached. Try to destroy the Wraith before it sets down, or kill it first upon it being dropped off. Then, rearm for the approaching assault coming up the ramp. Prioritize anyone wielding an Energy Sword, Needler, or charged Plasma Pistol. Careful with grenades, the marines can die to them. If the marines start to travel back down the ramp, focus fire on anyone approaching them. At least one marine might stay on the above ramp out of line of fire of the ramp, and if the rest die, fall back and defend him.
Third Stage- Phantoms
Once all of the baddies are dead here, get a new plasma pistol. DON'T give your marine a Spartan Laser or Rocket Launcher yet. This part isn't the most difficult, but can kill the marines much quicker with their now lower health. Grab a Spartan Laser yourself, and shoot off the main Concussive Guns from the approaching Phantoms. There should be only two phantoms on Heroic. Those guns should be gone before they arrive to drop off some Elites, but if they are not, forget them for now. Don't drift too far away from your marine as you kill the few remaining Elites that are dropped off, and don't die at all. After you have killed the Elites and other baddies, the hard part is over. Arm your marine(s) with Lasers or Rockets and kill everything left. The achievement should pop in game.
Bonus- If you still have a jetpack by the end of the mission, you can use it to fly up to your Evac Pelican and mount one of the side guns! Pretty cool :)
This is how i did it. Things may vary from player to player, but follow like this or at least take some advice from it and you should have it in one or two tries.