I’ve completed it twice, both times requiring a good amount of tries,
Ranged characters and one tank is a decent setup but a lot of stuff is going on all the time so being a bit tanky or good at generating tHP doesn’t hurt.
From the top of my head the spawns are as follows:
Go halfway down ramp/big open area to spawn the bile troll and a mixed horde + all the ambients. Soon after troll spawns there will be a ratling gunner and a flamer that might spawn at the heroes starting point + a few more ratlings and flamers spawning in the alley. The last danger in this wave would be the ungor archers spawning from the grim area.
Next two triggers are if you go to the first spawn points (down by bomb/ammo and upstairs (essence device level). It will spawn a couple of specials including ratling, flamer, hookrat and assassin.
4th spawn point would be just after the cave with ammo, leading to second tome, and it will spawn a second bile troll along with specials and a mixture of berserkers and other stuff.
5th spawn is very close to 2nd tome and spawns specials (assassins and hookrats largest threat) along with beastmen and a couple of banner bearers.
6th spawn point triggers in passage between 2nd tome area and the river or inside the caves. It will trigger a good amount of leeches along with a third bile troll. The same spawn or one very close after will also trigger a bunch of ratling gunners and flamerats + mixed mobs.
7th spawn? Not sure there is one.
How do you get it done? I would strongly recommend a tank with shield, BW and at least one other strong ranged class.
First priority would be to get a handle on initial spawn. Move halfways down the ramp to trigger it. Have one or two people kite the troll and most of the mobs, together or separately, up the ramp and to the first or second drop down (don’t go past the essence device). When the ungor archers have spawned and have been dealt with you should have a decent handle on things. The BW would most likely be kiting a good chunk of the mobs along with the troll at this point.
If someone goes down everyone should prioritize getting back to the first kiting loop between ramp and first drop down in order to get a decent respawn.
At this point you trigger the 2nd (and 3rd, if they are separate) triggers and do the first objective. One person can do can do this and also revive anyone who might’ve died.
These triggers bring new specials but not much else. Pulling back to the first area should be prioritized to handle specials and get rid of aggro/kill stuff.
When you once again have a handle on the situation and (hopefully) the troll is at least capped to 50% one person can run off and trigger the second troll. Both trolls should be dead before you proceed.
Next trigger is beastmen and you don’t really have to pull back that far on this one, but as soon as you’ve dealt with banners and specials + cleared some out you might want to push on to second objective.
If you don’t want the troll in the cave one person could take the shortest route to trigger the troll and pull back to 2nd tome area will the rest do the objective (should be done just before leeches spawn) and deal with the leeches in the cave.
At this point you probably don’t have much time left so the remaining three, preferably one mobility class and the tank, should move through the caves and use the river exit (where you enter the caves when playing festering grounds). It’s important that you don’t separate, tank will have to hold the rear, and mobility class should pick up last essence chunk (next to bridge - device next to bubble). The third person who is hopefully still alive will help clearing the way and dealing with ratlings/flamers.
I don’t remember much about the finale but you should be good if you make it to the bubble. You will be really low on time and you might need it all for the final fight, so be aggressive.
This is essay of the day. Hope it helps. I might’ve gotten plenty wrong about spawns and triggers but it’s the general idea. You need all three chunks in order to open the portal. You should be good on essence by killing two trolls and everything up until but excluding the beastmen. Whatever might missing will be little enough to get on the way to the bubble.