Through game exploration I had 4 scrolls of Meteor Shower and 2 scrolls of Hail attack. I also had a mage who knew the meteor shower skill and another who knew the hail attack skill. My archer had rain of arrows.
I gave the scrolls so each party member had 1 or 2. Scolls only cost 6 AP to use, so a couple of party members could cast more than one per turn.
Using only those skills, the game was over after the boss' first turn.
Optionally, make sure your scoundral has the grenade perks (never miss and longer range). And use the charm grenades that can charm multiple adds. This not only ups your DPS by the charmed adds attacking the remaining enemies, it makes the fight infinitely easier by reducing damage to the flimsy Astarte. Between charm scrolls, charm arrows, and charm grenades, you should easily be able to control adds... but again, with those master scrolls I mentioned above... you needn't worry about adds, the game ends when the boss dies. Hope this helps as one option to try for anyone still struggling.
