This is the currently the easiest way of killing the second boss in Azjul-Nerub and getting the achivement
What you do is you either bring a hunter or a rogue that range pulls one of the elite packs, then he/she pops vanish or feign death. At that point all mobs despawn including the boss, now you can just run down the hallway until you reach the spawn point of the boss. Basically you have more than 30 seconds without any adds at all (might be more) and even when they start coming its not that many at all for starters.
Another tip is to despawn them once the party is just outside the aggro range of the boss (if your slow running down and he spawns before your there).
Anyways pretty much any group can grab this without an optimal setup (my pet tanked it the last time we did it he he). This is even easier than doing it the normal way :jayne:
One might argue if this is an exploit or not but since its clearly not intended i decided on this forum.