I was doing some thinking and some reading and it seems as if the Zhaitan fight will take place in your personal story. If so doesnt that mean you will only get to beat him once and then have to play with your friends personal story to fight him again?
I dont think hes in the last dungeon because i read you need to join one of the orders in your personal story else you wont be able to continue your personal story which means you cant face Zhaitan. At least thats how i interpreted it.
Again i havent read anywhere that Zhaitan is the boss in the last dungeon but i have read that he is involved in the last part of your personal story along with one of the three orders you join.
So if you can only beat him once will that hurt the product?