Twice in a row one of the exploding adds that we have to kill was invulnerable to damage. After the second time, it didn't happen again and we were able to complete the dungeon. I submitted a /bug in game and this is the video.
V16 Uriel Stormblessed EP Magicka Templar(main)
V16 Derelict Vagabond EP Stamina DK
V16 Redacted Ep Stam Sorc
V16 Insolent EP Magicka Sorc(retired)
V16 Jed I Nyte EP Stamina NB(retired)