...try firing all of the turret lasers. These are the ones that initially spawn in the duel room, not the machine gun turrets that spawn later.
Backstory: I had a recent run where the mob spawned in but no Vardersoma. Group killed everything and backtracked, but we couldn't figure out why and 5 people abandoned quest. It was too late to do another 3rd run so I tried firing off all the laser turrets and lo and behold the big guy came down. Thank you other 2 ARKS who stuck it out and kicked butt in the final areas!