Just did the Act III quest with the catapults, all recruits stayed alive, but didn't get the achievment. What is needed to get it?
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:
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.
I did this one with my Demon Hunter without even trying, mostly through the use of Caltrops with the Jagged Spikes rune. I placed my Caltrops in the places where monsters were going to spawn before I started the event, then quickly started it before the durations ran out. On the second catapult, this killed all four of the big Hell Bearer demons before they spawned any demon troopers, which made that event just an exercise in waiting.