Once you get 3,000 points, go upstairs in the zombie factory place. 
When your 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. You will 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.