For this achievement you need to kill 3 bosses in Azjol-Nerub, explanations below.
Krik'thir the Gatewatcher
- Clear the three sets of adds in front of Krik'thir the Gatewatcher.
- Your group should stay close together and kill the Skittering Swarmers when they spawn.
- Only other noteable thing he does Curse of Fatigue which slows attack and casting speed. Have a mage or druid remove the curse and finish the fight
Hadronox
- The fight starts with Hadronox at the bottom room and you killing the three groups above, pull them away from where they stand.
- After all 3 groups are down Hadronox makes her way to you, do not engage her till she has killed all the left over mobs.
- Have your group stay away from the acid clouds and you should kill her fast.
Anub'arak
- Anyone in close range will be hit with impale and thrown into the air, so all casters stay back.
- Locust Swarm will deal damage to the whole party, so keep hots rolling when this is up.
- The tank needs to face him away from the party so they are not hurt with his Pound ability.
- He spawns adds, they need to be killed as fast as possible.