Hello, Do you mean on the Xbox or some other unit? I've wanted to play but never have yet before, so I don't have personal experience, but I found this information which should help: "To unlock The Devil Horns, you must first beat the main campaign. Then, you have to play over again with the New Game+ mode, but have difficulty set on "Classic." Once you beat Classic mode on New Game+, you should have the weapon." If you mean for someone else to get it and upload a save game or something, then sorry, I can't help you there. If you want to cheat at it via PC, that's a different story though. I'm new here so sorry if this isn't exactly what you're looking for, but I'm just trying to help.
