Once you finish the main portion of the game, traverse to the 50th floor and return to the room with the forge in it. You will see Kuhyra and if you talk to him, it will allow you to switch your guardian beast to Kuhrya, who can use mystic ore on all weapons. Also, the door which was previously "closed by a powerful force" is now open, making the final 50 floors of the labyrinth available.
After completing the game go back to the central tower and speak to the guy guarding the arena. He will ask you if you want to compete in an exhibition match for 1000 Boam. Say yes and you'll enter the boss attack mode were you'll fight all the bosses throughout the game in order.