At the very start of the map, before climbing down to the floor, head southwest to access a partially open-air area and get a chainsaw.
Flip the switch inside to complete the level. On the right side of the room is the exit. Continue down the path until you see a room with health bonuses, then turn left. In front of you are four Zombiemen looking back from you (be careful on higher difficulties as even a punch will alert more Zombiemen in the hallway).
