A Witch can keep up to three summons out of the battle with Blind, it's cheap and recharges every turn. I went with two, so she could still summon and do that huge damage spell. Probably could have taken the other two out as well, but I still needed characters to actually attack the boss.
After the initial round, keep your characters as far away from each other and Astarte as possible (while still being in the range for healing spells), so the breath attacks don't hit the whole group.
Always keep one or two summons around Astarte to defend her. She's completely useless during the fight, if you're really lucky she will occasionally cast group blessing.
