Note: this summon is optional, and you can come back any time to challenge Odin if you find it difficult at this point.
He is in the basement of Castle of Bal. However, you'll have to enter the basement via Jachol Cave.
Boss: Odin
You have exactly 1 minute to defeat him. During the battle, he'll probably use Zantetsuken once, which could kill some of your allies, so having high defense is recommended.
Strategy 1: 1 Time Mage and 3 Black Mages. Equip Flame Rod and Magus Rod on the 3 Black Mages. During the battle, all Black Mages keep spamming Firaga, and the Time Mage's first turn should cast Hastega, then follow up with Comet (you can also use Firaga if you have the Black Magic ability). With Flame Rod equipped, you should deal close to 3000 HP.
Strategy 2: Odin is susceptible to Break. This can be Spellblade - Break (Mystic Knight), Break (Black Mage), or Catoblepas (Summoner). So, you can mix up these jobs and keep spamming Break until it sticks.
Strategy 3: Zeninage (Samurai) also deals around 3000 HP, but it costs money. You can have 3 Samurai to quickly kill Odin.