To put it simple, he can kill multiple enemies with a barrage of AOE spells in rapid succession (which should be impossible).
Thanks to him, the girls are able to experience the dungeon every day with very little danger, and they are able to quickly kill bosses in a few minutes, something which would take other parties 30 mins to a few hours.
Also, they progress up the dungeon in a unbelievable speed, 1 floor per day, which again should be impossible, since it would take most groups weeks to complete a single floor.
Yet, he is only able to do this because the girls keep the enemies from rushing him.