Instructions
Load a saved game, preferably one on the second playthrough (or higher). Travel to New Haven and open each of the six weapons crates, using the New Haven Weapons Crate Map.
Open the map, then fast travel to "The Back Door." This level is on a straight path, so finding each of the Weapons Crates is straightforward. The other advantage of this stage is that there are two mini-bosses that can be fought whenever a save is loaded. Farming bosses can drop unique or legendary rocket launchers like the Mongol.
Enter the "Crimson Fastness" and complete the dungeon, using the Crimson Fastness Weapons Crate Map. In total, these three areas have 15 Weapons Crates and easily killed or avoided enemies. Upon completing this dungeon, save the game and exit to the main menu in order to repeat the process as many times as necessary.