The way we would do this achievement is we would stack on the Reaper at the start, and assign 6 tanks, one for each of the flame and frost assaulters. We would DPS the reaper to 96%, then hold until snowblaze does her frost AE rings. Once the rings went out, we would kite the frost assaulters in the eastern area, right near the tunnel toward where you zone in. After the Flame assaulters shield was broken, we will kill all 3 of them, and take Reaper to 89% to rebalance them.
The Emanating Flame AE is the strongest in the eastern area, near the tunnel to zone in and the scaffolding. We would keep the frost assaulters over there and soft tank / kite until they had enough AE's to break their shields, then we would kill them.