You can't have the Black Demon serve as instructors, then let them be sparring partners.
Sparring partners for athletes are called sparring partners. For soldiers, they must be called the opposing force.
If any unit wants to improve, and real combat isn't an option, then they must conduct drills now and then. Drill exercises can't just be target practice, nor can they always be performed behind closed doors. This is where the benefits of an opposing force become apparent.
Wolfgang commands Israel's strongest Special Forces and sees there's something to learn from the Satan Mercenary Corps. He even tries to keep Satan around to serve as the opposing force.
If you want a knife to be sharp, you must have a whetstone.
