Go south first to pick up an Antarctic Wind, which may be handy against the boss. Djinn is weak against Ice, so if you picked up the Blizzard Spell in Castle Sasune, now is the time to use it. Note that it does have some Magic Defense, so the Spell can miss on occasion. This will be a long fight, so be ready with Cure magic or Potions. When the fight is over, you will automatically warp back to Castle Sasune.