The Tower of Owen is a multi-story dungeon that requires the Toad spell to navigate. You'll need to transform your party into Toads to pass through certain small openings. Remember that in Toad form, your physical attacks are severely weakened, and you can only cast magic. Plan your job setup accordingly, ensuring you have strong magic users. The tower is filled with various elemental enemies, so a balanced magical offense is beneficial.
At the top of the Tower of Owen, you'll encounter the boss: Medusa. Medusa is known for her ability to inflict the 'Stone' status, which can instantly incapacitate a party member. Equip items that protect against Stone if you have them, or be ready with Gold Needles to cure it. Medusa is weak to Fire magic. Have your Black Mage cast Fira or Firaga. Your White Mage should prioritize healing and curing Stone. Physical attackers should focus on dealing damage, but be wary of her gaze. Defeating Medusa will allow you to proceed to the next area, unlocking the Scholar job, which is excellent for identifying enemy weaknesses.