Hi, first of all:i used the search function and couldnt find something about it, so i dont think its a repost.
So here it goes: Right after you killed the first boss in Azjol-Nerub, you have to go to the platform, where you can see the 2 stairs, where the mobs are running to hadronox.
Most of you now its a event i think and if you die, they will respawn after about 15 secs. If u are a Hunter, you can just aggro the first group with your pet, let them kill it and feign death - the mob group, Hadronox and even the little spiders from the stairs will despawn.
Now you have about 15 secs to run into Hadronox room and just kill him like always. After you killed him you will get the heroic achievement for killing him without blocking the little spiders from the stairs to spawn.
I think if you have a good rogue or mage, this will work fine too with invisibility or vanish. But very important is, when you pull the mob group, no one of your group should get aggro (just stay far away from the group) else they will come kill you instead of despawn.