Game in a nutshell: Grab God of Death (yeah that is its name) rifle from item box, may require a few tries. Hop onto a staircase railing and one tap all zombies who have awful pathfinding aside from the boss one which you can easily kite since it seems lost. Rinse and repeat for as long as you can tolerate it. I unlocked roughly half of the scenes (a set is unlocked after each boss wave) but it is just an absolute slog.