To build it you will need a total of 2500 whatever the currency is on Black Ops 2, but that will let you open three necessary doors. To get to the rooms where the pieces to build the weapon are, you need to take first the broken elevator from the first room, where you start the game, and keep going until you jump to the next building.
One of the pieces could appear where it appeared on the video, right in the kitchen, but that same piece could also be on top of a container next to the stairs shown in this video where the building room is. I want to point out that you don’t need to turn on the power, it’s just not necessary.
The first piece is right next to the building table. It is an extinguisher and it is in a cage. Press X to take it and press X again in front of the table to start building the weapon.
The second piece, and before it is important to say that the order does not matters in this case, so I was saying that the second piece was in the kitchen but it could also be on top of a container when you go down stairs to the building room. Do the same process until you finish with the two remaining pieces.
The third part is in the power room. When you enter the room, turn right until you get to a table before the elevators. Take it and add it to the weapon.
The fourth and last part is right when you go upstairs. It is in front of a tv laying on the floor.
Now that all the pieces are together, take the weapon and gather as many zombies as you can. They need to be very close to each other, otherwise it won’t work.