The humans won the battle.
The assistance of the mysterious group of fighters who broke through the side ranks and slew their way toward the center created a critical collapse to the battlefield.
As the soldiers had pushed back the forces, they had the rare opportunity to pull back all the slain forces to help their forces level up!
Since the corpses were often rich sources for level-ups, it was common practice that an army would retreat at a vital point of the war so that the deaths left behind would not be too many. And platoons would often harass the victors to ensure the army would not return the corpses left behind.
After all, if the army won the war and acquired the corpses, it meant level-ups for the enemy and could raise the quality of the soldiers.
The imps had always been able to harass the humans as their Krampus could cast ranged magic.