Minutes to Midnight

Enter Dragon Soul and engage Ultraxion. At max level you can solo this encounter, and if your damage output is high enough, Ultraxion may only cast Hour of Twilight once before dying, completing the achievement automatically.

Hour of Twilight is cast every 46 seconds, meaning you can face up to 7 casts before the hard enrage at 6 minutes. The majority of the raid must use the ability to phase out of the twilight realm to avoid each cast — any player hit by Hour of Twilight more than once disqualifies the achievement.

Each Hour of Twilight cast requires a specific number of players to remain in the twilight realm to prevent the Aspect NPCs from dying: on 10 Normal keep 1 player in, on 25 Normal keep 3 players in, on 10 Heroic keep 2 players in, and on 25 Heroic keep 5 players in. Rotate which players stay so no single player stays more than once across the entire fight.

Tanks can use their standard defensive cooldowns to survive Hour of Twilight. Non-tank players staying in will need their own cooldowns: Death Knights can use Anti-Magic Shell, Icebound Fortitude, or Anti-Magic Zone (Unholy); Feral Druids can use Survival Instincts or Barkskin; Paladins can use Divine Protection with Charred Glyph or Divine Shield; Warriors can use Shield Wall; Shadow Priests can use Dispersion; Mages can use Ice Block; Fire Mages benefit from Cauterize; Hunters can use Deterrence; Rogues can use Feint or Cloak of Shadows; and Holy Priests can cast Guardian Spirit on any player staying in.