Defeat the Boss Monster.
There is nothing remarkable about fighting the Boss Monster though. He normally enters the stage wielding a lead pipe, which can easily be disarmed from him, along with a small band of normal and stout zombies. Use weapons lying around (most numerous at this point of the stage are pieces of wood, boxes and pistols) and your Anar-Chi move as much as possible when directly attacking him while avoiding to get overwhelmed by the zombies.
The Boss Monster needs to be finished off when his health drops low enough. He will get back up unless you smash his head into ground with your boot. Zombies can interrupt your finisher if you aren't quick enough though.