Hirge will use Might at the most unusual of times. Against Level 1 Goblins, against Fire elementals, against the waiter who spilled her Friguld rum. It's a side effect of her AI.
To mitigate her Might problem, you can give her a 2-H weapon for some fights. The extra physical damage will off-set the fire healing. Or you can just plan on her having a reduced ability in the fight.