I did a challenge run on VeryHard, with no skills equipped and no consumables on the girls, and discovered that the final boss fight is impossible to win in this challenge because you run out of ammo for both weapons with no way to replenish them. I've managed to bring the boss health down to 1/3 or even 1/4 but that's about it. I guess it could be possible to spend an hour slowly grinding the boss's health down with the pistol, but the issues are the tall zombies and titans that will constantly spawn and the girls will get literally fucked failing the mission.