All the spirits of the Demon Lords that Mahazael summoned were no match for the Valkyries. In fact, Brunhild alone was enough to deal with all of them, and she was unscathed from the battle. She slaughtered all of them in a single swoop with her spear.
Although Brunhild was enough to defeat them, the other Valkyries didn't show any mercy. They took the battle as their last battle as Brunhild taught Gunnar, Vincze, and Sven. They remembered Brunhild's words that in war, no matter how weak or strong their enemy was, they needed to go all out and put everything on the line. Those words were carved into their brains and always went all out no matter who their opponents were.
It was a slaughterhouse, and even Mahazel who didn't have any emotion or feeling was terrified by how powerful the enemies were. He summoned more and more demons into the room to fight the Valkyries, but to no avail, all of them were slaughtered as soon as they were summoned.