After reaching 3,000 points, go upstairs in the zombie factory place. While upstairs, if you used the main steps on the front start area, turn around. Go around the steps and go back to the back of the room near the sandbags until you see a vanity or dresser object behind a pillar. Go up to it and open it to get a flamethrower. Go around the pillar over to where the steps are located. Turn left and to find a room and a wall in a protected corner. Go in the corner and use the grenades behind you and your flamethrower to kill.