The need to skip a turn at the end is a workaround. If you give the victory immediately after Bertrand's defeat, the game will crash under certain circumstances. Unfortunately.
Against Bertrand, the Staff of Sar-Issa is effective, which is given in the hut in the deepest parts of Benedict's domain. It is assumed that the main boss of the dungeon is Vessel, and Bertrand is just there to give you a hard time, but sometimes it happens differently)
Thank you for the walkthrough)