Diablo – How to get the “no man left behind” achievement

I did this on Nightmare with a Monk, and one thing that I noticed was that I didn't get the achievement until after I'd raised the third catapult, even though there's no recruits that can get killed past the second catapult raising.

The recruits can also be killed before they start to raise the catapults, so it's possible they died offscreen before getting to the winches. It's also rather hectic during the mission, so it's possible you missed someone dying in all the unrelated slaughter.

Once the recruits have finished raising the catapult, they'll follow you around for a while. It doesn't matter if they die past this point. They've got crazy health though, compared to how easy they go down during the event. I once watched one tank an arcane enchanted plagued champion pack for longer than my Templar...

Since you're likely to have to attempt this again, I'll give some general pointers:

• Before talking to the NPCs near each catapult, completely clear the area and scout a short ways ahead, in order to avoid missing any enemies that spawn before the event starts. A couple of times, especially at the second catapult, I've come across an elite or champion pack that tends to get pulled into the fray. This can really ruin your chances.
• Make sure you maintain aggro - hit new enemies, and don't remove yourself from play by using skills like Spirit Walk, for instance.
• Bring as many pets, followers, or other players as possible. It's really a matter of distracting the enemies - they will only tend to hit the soldiers when there's no other target.
• I found it easiest as a melee class to stand near the recruit on the middle winch - this gives you flexibility to aggro incoming enemies from either of the two sides.

Clearly, being able to do significant damage quickly makes this easier, so dropping the difficulty and getting good DPS weapons is a plus.

At the second catapult raising, try to kill the monster spawners that are in the lower right of the screen quickly, as they tend to appear and drop enemies a few seconds before the ones in the upper right. I found that frequently I could take them out before they even spawned a single wave of enemies.

Don't abandon your post at the middle winch to take out the other two until the catapult is raised, however. Prioritizing these spawners when there's active monsters about is a sure way to lose.