Odin Summon Location and Guide
If you've never been through Jachol Cave before then let me explain how to get past the first room. In the southern hallway you'll notice a long line of skull shaped switches, if you stand around long enough you'll see every switch vanish along the wall except for 1. When this happens you'll want to run over to that one switch and hit it. This will move the wall and allow you passage.
Inspect the opened Treasure Chest up north to open the doorway and proceed further. At the end of the cave you'll find a dead end... but not really. Approach the wall to climb up it and find yourself in Castle Bal's basement. If you spent some time grinding in the location I talk about in World 2 you may recognize where we are.
Once you're in Castle Bal's basement head north through the room dodging the cracks in the floor until you come across a small fire inside of a green bubble. Heal up and once you're prepared for a fight, inspect the bubble to summon forth Odin.
You will only have 1 minute to defeat Odin who has a total of 17,000 HP. Unload on him with your most powerful Summons as well as XAttack/Sshot abilities for anyone who you've Mastered the Hunter class with. Typically you'll defeat Odin with 5 seconds or so left on the clock. If you're struggling to beat him, the same room that Odin is found in is a pretty good location to grind both ABP and EXP.