To fight the Behir you need a fully upgraded Sword and very high Strengh (12+) + Defense (24+). The Downside having a Hero that invest only in Combat Attributes: You will miss several Attributes checks. The Mole Dungeon rewards Players with Charisma (7+). Meanwhile the Behir demands a very combativ Hero.
Maybe just settle with diffrent Stats on diffrent Playthroughs. Or use Incense Burners to reset Stats before the Dungeon.