Tried that tactic last night and all 4 of them came after us at the same time, We got a AOE dot spamed on us too with 2 clerics a PBT thurgy a Armsman and a Scout, and we got smeared in less than 1 minute.
Tactic my group used was having one person drop from group and pull his demonic minions each separately. This worked great on two of them, but it appears that the last one will always be stuck to Aogan, so you have to fight Aogan and last Demonic Minion at same time. To avoid having the minion healing Aogan try to pull them as far apart as possible having a pally or other tank hold agro on aogan and have rest of group on minion. Once minion is dead everyone on Aogan. As soon as Aogan dies Lord Bile will spawn immediately and in the same spot where Aogan died.