Ascendant Descending

Enter Blackrock Caverns on Heroic difficulty and fight your way to Ascendant Lord Obsidius. At max level you can solo this encounter, and nuking Obsidius down fast enough before stacks accumulate is the simplest approach.

Ascendant Lord Obsidius is accompanied by 3 adds that channel a beam granting stacks of Crepuscular Veil to anyone in melee range — the beam cannot be interrupted and continues while the adds are moving. The adds do not attack by highest threat; they target whoever most recently generated threat, and they can be snared but cannot be rooted or crowd controlled.

Assign one player to kite the adds in a large circle around the boss, staying roughly 30 yards away from the group at all times, and never running the adds through the rest of the party. A second player can periodically run out to snare the adds to help the kiter maintain distance, while the remaining players focus damage on Obsidius and taunt him after he swaps targets.

If playing a class with a ranged pet such as a Frost Mage, keep the pet away from the adds entirely, as the pet passing through the adds triggers Crepuscular Veil stacks and can cause the achievement to fail.