Get to round 25 on Ascension. Go to the mystery box and go prone for 5 seconds in front of it.

Then buy a gun from it. Use a lunar lander and do not collect your gun!

End the game and select multiplayer from the menu. Choose private match and go on your file share.
Save a random film.

Go back on zombies and die at round 3. Go on multiplayer and render a film.

Choose Demolition and you will have the Raygun.

[unconfirmed]