Phase 1

The fight will begin with only Attumen's mount, Midnight.
He is neutral and will not attack until the party initiates the fight.
Have the off tank engage Midnight and the main tank stand close by, preferably somewhere between where the casters are and where Midnight is being tanked.

A few seconds after the party attacks Midnight, Attumen will appear and attack the party. The main tank will then pick up Attumen.

The raids DPS will be focused on Midnight while the main tank holds Attumen security near the wall.
Both Midnight and Attumen only have basic melee attack with the exception of Attumen’s debuff which will require a dispel on the main tank.
Rogues and other melee may want to note that Midnight has a "bucking" attack which hits players standing behind him.

When Midnight reaches 25% phase 2 will begin.



Phase 2

All aggro is wiped and our main tank will need to immediately reestablish aggro from Attumen.
All DPS will then focus on Attumen while the melee DPS beat away at Attumen’s back. Anyone within melee range will get cleaved and require a single heal or bandage. Attumen will also randomly charge ranged party members and return directly back to the main tank.

Buffs

Amplify Magic on Off Tank
Shadow Protection on the Raid
Curse of Weakness on both Attumen and Midnight

Main Tank / Off Tank
Phase 1: Maintain agro Attumen / Maintains agro Midnight
Phase 2: Maintain agro Attumen

Melee DPS
Phase 1: DPS Midnight
Phase 2: DPS Attumen

Ranged DPS
Phase 1: DPS Midnight
Phase 2: DPS Attumen

Healing Classes
Phase 1: Share the healing focus between MT and OT (keep debuff off MT)
Phase 2: Heal on MT/Remove dot off melee/Watch for charged raid members