It does work, but if you can't kill him before any boulders spawn you need to make sure you open the chest IMMEDIATELY as the boulders don't disappear - they will keep rolling around while you loot the item and open the chest. If it hits someone, you'll fail it.
I burn down last mob and got the achievement before last month patch/ but now burn down method is not working anymore? you are saying, I need to open the chest while boulder still rolling around?
Nope, it still works burning it down. Are you doing it for someone that isn't in zone by any chance? If so, they MUST zone in before you loot the mold and open the chest or they won't get the ach.