These Demon Army commanders are pretty hard to kill. Wiz took in Vanir as an assistant in her shop, much to Darkness' embarrassment after saying cool things about him.
Part of the RPG experience is upgrading weapons and armor. This being Kazuma's world, he ended up not being able to use either new armor.
Kazuma thought his plan and contingency plans were enough to beat the quest. He forgot to factor in a goddess with impulse control issues and his own bad luck outside using his skills. Yeah, he died, again.