This Odin is a dark attribute knight of remains. He has an armor that protects him from all attacks. This means that no matter how hard you hit him it will just end up being one damage on him. So what does a person to do to win?
Well the answer is quite simple.You'll have to combo 5+ runestones to get enchanted runestones and this is the only way to deal more then one damage to Odin. The only way to win is to use enchanted runestones. Skills that deal damage directly (even ones that disregard defense or consume enchanted runestones to deal damage) will still only do 1 damage to Odin, so keep that in mind.
Now that we know how to attack him, we need to know how to defend against him. Odin when he attacks, which is every THREE TURNS, deals 3900 damage to you. If you are not properly prepared, you will be defeated extremely easily. I would recommend having a friend Eternal Granter Idun or another monster with healing each turn, as this will heal you each turn. You can also get an ally that has a 30-50% damage reduction to dark.
Lastly, the final key to beating Odin is patience. This is due to the fact that you will need time to get the enchanted rune stones in place. That is all there is to do it. Good luck in your seal breaking!
Create enchanted runestones to damage Odin, Get an ally that can either heal you every turn (i.e any Idun) or a dark damage reducer (i.e Decapa). If you have Gods that either convert regular runestones to enchanted ones, or create more enchanted runestones (ie. Poseidon), they will help in the battle as well.
A good strategy to defeat him is to form as many enchanted runestones prior to entry to the boss fight. Keeping 4 enchanted runestones on each of the bottom corners allows you to maintain an attack per turn so long as you match 1 of those with 4 non-enchanted runestones. This will allow for a continuous flow of enchanted runestones.