Beat the game once and make a clear save. Reload any save before the last boss and go to the world map.
In the World Map there is a place called the Old School Building. A sub-section of this is called the forbidden zone, there is a teleporter deep in the dungeon. This teleporter leads to the bonus dungeon.
What awaits you in this dungeon:
- Very powerful monsters with different elemental nullifications including physical type attacks
- Stronger version of the Final Boss
- Boss Cameo from Atelier Iris 3