Besides your priest, do you have another character near Astarte to do damage to the summons (if convenient, the main character that Zandalor follows, so he sticks around there, as well) as well as take some of the attacks that would otherwise be focused on her?
Do you have summons to go after the void wanderers, and hopefully take out a couple, or at least keep them busy while you focus first on the melee ones around Astarte?
When the conversation starts before the battle, you can separate the other characters and have them position themselves, and, if you are quick, maybe get some buffs or summons going before switching back to the conversation to end it and start the fight.
