This is a tale back from the time when humans and demons fought over the world.

The demon king commanded not only the common demons, but also, the so called "mid-bosses", demons with considerable power, that he deployed in strategic points.

However, these mid-bosses kept getting defeated by the heroes, who continued to grow in strength with time. Which led to the demon king's army needing to produce even stronger mid-bosses.
It was at that time that a certain demon stepped up.
Demonstrating great wit, she became the founder of the demon world's mid-boss training academy.
(from WhimSubs)