I throw a grenade and sprint back to the left corridor (or through the center to shoot each turn?), and for the next two turns, I run to deal with the demonologist. Ember retreats to the hall – I hope she's not slower than the undead, otherwise, it's unlucky. If the necromancer doesn't enter, Denym and Beretta will cover Ember and hold back the undead at the entrance to the hall. And if she enters, we need to keep the weak-willed champions away from her and take her down as quickly as possible with the assassin or shoot her with the rifle before she causes too much trouble, including with the guests.
Merette and the archer should start by shooting at the small demon.
I asked Revan some important questions, but for now, this is the plan I've come up with.