Yamata no Orochi
The demon nest that the so-called siblings were dispatched to was considered to be a medium to low-level one according to the intel that was extracted out of Crane's brain. As such, the commander of their team wasn't exactly that powerful as a Tier-8 Halberd Dominator.
On the other hand, the enemy commander was a notch more powerful than that, almost equalling Tier-9 powers, which caused the human commander to be always pushed down and forced to use multiple cards and moves to keep up.
Thanks to that, even after the major weakening of demon commanders took place all over the domain, the human commander was in a tattered state that didn't allow him to decisively take care of the demon commander, as he was finally cut some slack after the battle that was full of tensions and close brushes with death.