There are three wizards (one of which is the deathsinger), a shit ton of Knights, and two shriekers. A single wizard needs to be drawn out on both sides and then killed. By the time this happens, the Deathsinger will (or should have) started her song. It's at this point that a shrieker will appear on either side of the crystal room and those need to be killed in order to knock down the shield (similar to the hallway). At that point, the fire team should DPS the deathsinger and kill her. If too much time goes by a 30 second timer will start which will cause a wipe when it reaches 0. Once the deathsinger is dead, the remaining knights need to be killed. Acolytes will continue to spawn in as long as knights are alive so ignore them and focus fire on the knights, if you can. With a full team, it shouldn't take that long to complete. If you're doing it with less people, it take considerably longer.