Instructions
Complete between three and five rounds in the first area you are fortifying against zombies, depending on how many points you want to accumulate before moving to a new area. Points are used to buy weapons, ammo, boosts and unlock new regions.
Open the double doors in the initial room your team starts in. Walk up the stairs in the next room and remove the barrier there, which costs more points.
Walk to the mystery box in the attic room at the top of the stairs and hold the "square" button to purchase a special weapon. You may have to try a few times to randomly receive the flamethrower.
Hold "square" again to take the flamethrower, which has unlimited ammunition.