I have boxed this mission successfully once or twice, with about 30-40 failures
I took the slow approach to the process for the mission to first of all learn it, then hopefully perfect the mission. I did have some issues tryign to get all of the toons to zone over to the rooms as needed, so chose to just zone over the tank and healer in most cases, and then slow DPS the boss down. I did not go for the greedling achievement at first, but I am tryign to find a good way so do that achievement also.
Here are my thoughts and process so far (though it will definitely evolve, and I may still have some elements incorrect)
1. Kill echo down to 80%, he goes inactive.
2. Zone 1 toon over (normally the tank) to the North room. I follow the NEWS route (North, East, West, South) to process the rooms.
3. Put the flagellant on extended target, then zone back to main room and watch him kill himself. This is the most boring part of the mission.
4. Kill the echo down to 60% . If you want the greedling achievement, do nto really DPS the Echo down, you want the greedling to spawn and give him 100pp 3 times I believe to get the achievement. Still learning this one.
5. Zone to East room to kill the Dark Oppressor. I didnt really see any mechanics, though he did a bunch of emotes as I slowly killed him
6. Zone back to main room, and kill echo down to 40%. I slowed DPS to get the add down (grudgeling) also before the Echo went inactive
7. Zone to the West room, and kill the Dark Maniac. He has a charm component, but with only the tank and a shaman in the room, it did not really affect the toons very much. With a DPS toon in the room , it may act differently.
8. Zone back to the main room, and kill the Echo and the add (spiteling). Same issue with slow DPS on Echo while killing the add first.
9. Zone over to the South room to kill the mob. I dont yet have the mobs name, since this was the least visited room in my attempts. This is the one that requires all toons to be in the same race. I used skeleton illusion from different sources, and it worked for all toons. AON, Frost bone illusion from an enchanter, etc all seemed to work for this. I have also heard a campfire illusion will work, but have not yet tested this
120. Zone back to the main room, kill echo down until he reset to 100% and seems to port everyone to a different room. I seemed to have some mesh issue, but I wasn't sure as it all happened pretty fast. I DPS'd the Echo down, while watching for the oozes, moving everyone around the room to hopefully avoid them as they slowly moved around. Killed the Echo, all oozes despawned.