Firstly, you'll need a Black Mage with you. Anyhow, inside the Lunar Ruins, during Cecil's trial, you will eventually get to fight Lunar Odin.
At the start, use Slow on him. For the next few turns, focus on keeping your party alive. After Lunar Odin's sixth turn, on which he'll cast Holy, have your Black Mage cast a Thunder-based spell (Thunder, Thundara, or Thundaga) on Lunar Odin. With good timing, you'll hit Lunar Odin on the same turn as "Lightning courses through Odin!" appears on-screen. Assuming Lunar Odin is hit with the spell, it is an instant-KO.