I actually got this on my second try, and the first try I wasn't using an optimized setup. Once you know how to do it, this level is more annoying than hard.
Make sure you are running on the lowest resolution with all the video options turned off (0% particles, no numbers, etc). The improved speed is important!
Ketta: Use the cold bow and maximize the bleed skill. Ketta's main purpose is to slow down and place damage amplifiers on the spawns.
Niru: max bite, epic halitosis, nom, and roar. Everything else is irrelevant
Wrenna: You should use your standard Wrenna build (max the boost 4 and 5 skills, use beast's weapon, assign the rest of the points however you like)
Place all your people either near the entrance or on the track at the bottom left. Put Niru adjacent to Wrenna, and turn off all his skills except roar and bite (I guess you can use claw, but its DPS is so pathetic I don't bother).
For the first 40 levels, things just die. For the next 20 levels, a few things sneak past unless you use spells, but it's still REALLY easy. For levels 60-80, you will want to start moving Wrenna and Niru around to kill the worm spawns (from when the monstrosities die). Starting around level 80 (when the monstrosities from level 75 die), it's probably best to move your whole team around to kill the worms... just make sure you are back to your standard kill-zone for when the next monstrosities come around.
The Mad Ghouls that spawn from the Cultists are annoying, because they can attack, and by the end of this level, they hit REALLY hard. Spam lightning on them and replace people they kill. If you are worried about psi, just move your people to stay out of melee range of the Mad Ghouls.
Niru with max nom can insta-kill an enemy with 18% life left. There are times when it's a good idea to just put a 1-boost Niru where all the enemies are and watch them die. Her roar is also outstanding, especially when you place her near Wrenna.
Just Challenge 5 left. Although, I don't know when I'll be able to get to it!!