Like most enemies in the Ancient Castle, it has high Magic Defense and inherent Reflect. It can use Freezing Dust, which inflicts the Frozen status on a character, and Meteor when left alone. Fortunately, Lunatys like to attack in groups. Summoning an Esper or using multi-target, Defense-ignoring spells such as Ultima work well. They are also vulnerable to Lv 5 Death, so if Strago is in the party be sure to use it against them.