I'm not a fan of posting others' guides (then again this is the first time I post a solution), but after noticing that the only other solution for this achievement can result in frustrating trial and error, I had to share this. With this method by Life Expectancy
, I managed to have 15 miners by the time I entered the elevator and could afford to have many of them die with no consequence to my achievement.
To add to what Life Expectancy
does in the video, I found that a good way to get two miners in your Warthog(which he had trouble with) was to simply command your Spartans to go somewhere far away. The miners follow you around, but they do not follow your orders, so while the Spartans are away, you can get the miners to cooperate without the Spartans getting in the way.
Another thing: Try to rush the Phaetons while leaving the miners behind as far as possible, for obvious reasons. After crossing the bridge, particularly, you can go around to your right and wait for all of them to follow you to be kept safe. Once done, you can peek out and shoot that spot's Phaeton from a safe place. If the miners go out into the wild again, just rinse and repeat.
EDIT: I was playing through the level again, going for the skull, and decided to take this screenshot. This was on an Easy playthrough, but the point is to give you a visual of the large number of miners you can save. If you're like me, you probably didn't know you could save so many at first.