There’s really not a lot of dmg coming in unless your group is failing mechanics.
The illusions spawn as neutral. Not sure why people are saying to tab every once in a while, tab targets do not work on neutral targets, unless it’s a setting you have to change (plz enlighten me). I do suggest pressing the “v” button and turn on enemy name plates though for this fight to help see any new illusion spawns, and everyone should stand on top of tank to ensure any illusions that spawn that attack you, that you can see them and target right away to kill. For priest healers, it sounds like you want to switch to your mind spike rune, I heard that is a good one for them to take out their illusions quickly. If the tank is getting low on health and you have illusion on you as a healer, ask in comms (you should discord for kara runs) or type worse case if you have a dps class that can help spot heal the tank for a moment.
Even if you have a hunter with flare in the group, I recommend everyone to stand on top of tank with the torch and only let a range hunter stay out as sometimes they move/have to redo flare and it can increase the change of missing an illusion spawn.