My experience with the last fight. He splits into 4, a number of times during the fight. If i remember correctly once at around 90%, once at around 50%, and again at around 20%. these aren't precise, i'm sorry.
When he splits you pick one and dps it down to around 65%, at this point he MAY get a small blue aura. If he does switch to another immediately. If you don't get a small blue aura, kill it and repop the main mob.
If you see the small blue aura and keep DPSing, it turns into a large blue aura making it VERY difficult to see auras from the other mobs. If you kill a mob with a blue aura, it will DT your group.
If you dps aura mobs past the point in which they show you the aura and don't kill them, once they regen they will NOT show you an aura again. For instance if you kill and aura mob and get DTed, then maybe someone survives and kites them until you can reengage. All the mobs that had been DPSed far enough to show an aura will not show an aura again. At this point its pure luck as far as you getting the right one.