You have 90 secs, at most. Shadow aura does like 2k dmg per sec so average raid HP = 120k. 120/2 = 60 secs. Not counting resists, barriers, shields, etc...
If you use 2 discs things are easyer. Just make 1 disc start from g1, and the other from g5. LK style. Don't bother on shielding targets with aggro. It's useless. Really. Save all the intervates for those discs. Make everybody be stacked in one point. If melee needs to hit from behind make 2 groups. Melee and the rest, and throw them a barrier as well. Save BL and haste pot for this phase. As I said, if the tank can resist at least 10 - 15 secs ( 2 or 3 hits), the boss should fall.
The most important thing here is having the raid HP topped before phase 2. Make them use HS when the boss is gonna change.
CibÝ - GM of D N A, EU-C'thun