I finally managed to get through this encounter cleanly, without any party member falling or any paladin getting killed.
I got close enough so that I could see the NPC's, without being forced into combat. Guess what, because I was far away enough, the NPC's were actually invulnerable and could not be harmed by the Voidwoken. So I waited a bit until all the Voidwokens had taken their turn, then had only Beast go in exactly at the same time the paladins started to take their turn, and tank all the bullshit the boss Voidwoken promptly threw at him. The fight went much more smoothly. Sometimes you just have to resort to doing things like this if you're too much of a perfectionist.
I must say I'm still eager to see what other "surprises" the DE has in store when I get inside Arx proper.