Santa Slays Slackers question

[Jostabeere]

So I feel I don't get the achievement.
What does the game mean? Keep the 4 working elves alive for 30 minutes after the first assault? Can they die?
Or is it rather: "Keep them working without cops interrupting them"?
Or is it completely different and I have to keep the hobos alive that are infront of the workshop?
or the thugs?
I am confused and I don't wanna tackle it without knowing what to do.

[Anchored at Hell]

if you let cops interrupt them for a while, they will be gone/removed, achievement is all about preventing that