Compared to the other bosses on this floor, Astaroth is fairly easy. If you have a party member that knows Reflect, or even better, two, reflect all party members instantly. Astaroth's main threat is her magic, which can easily be reflected back to her. She also has a physical attack which drains your HP and heals hers, however it's not that strong and shouldn't prove to be a problem. She uses Firaga, Blizzaga, Thundaga, Death, Flare and Embrace, all of which can be reflected. As long as you don't run out of MP and can keep casting Reflect on party members, this fight should be a breeze. While your mages cast Reflect it might also be useful to note that she does not counterattack, so you can attack with your warriors to whittle her HP down faster.