This is weird. For me the dragon takes his turn first and focuses Astarte, but never kills her in one turn. He gets her to maybe half health and then I just heal/cast shield on her and everything is fine.
In your game is Astarte damaged from the battle with Trife when the dragon fight starts? Does she have any debuffs on her? Also you know you can delay starting the fight with the dragon as long as you want, right? It only triggers if you move up far enough. If need be, you can just stay out of combat and pre-cast some shields on Astarte and then trigger the fight.
