Each time Odin is defeated, he'll be stronger the next time he spawns till he reaches his lvl cap of 60. Each time he uses Zantetsuken successfully, he'll be weaker the next time he spawns. If FATE times out, he will spawn at same strength the next time.

Also as someone said before, he has a move that hits super hard but dmg is split, so need to stack people, preferably tanks to survive it.

The fight is meant to be done with multiple parties. Although usually any kind of strategy is difficult to implement since there are so many people, you run into rendering issues, some people getting frame rate loss, and you also have multiple tanks fighting for enmity.

I've done the fight as all roles and main difficulties I've seen are:
DPS: Keeping him in range/frontal arc, dodging cleaves when he's constantly spinning around.
Healer: Keeping everyone topped off or reviving dead people, when half the party is untargetable due to render cap.
Tank: Same as dps if you're trying to OT. If you're MT, having multiple tanks trying to take enmity (can understand if current MT is undergeared and struggling, but no real reason to keep ShO/Def up if current tank is handling Odin just fine).