The only thing that could be better than one amazing B.F.G. is two! You should always have a backup as an Evil Overlord! A Backup plot a Backup gun and of course backup controls for those days when someone sabotage’s the primary control panel. Preferably something you keep upon your person. Have twice as much as your advisors tell you to have and you will have a long and prosperous reign.
History teaches us that not having redundancies in place will cost us. If the power coupling goes down… there should be a secondary. If the first pistol jams there should be a second, and don’t forget to have the armorer killed for shoddy workmanship. If the primary control center goes off line then switch to the second control room. Same goes for power plants. If all of your power comes from one source you are vulnerable. Multiple sources multiple sites and even the secondary places should be guarded and fortified just like the primary site.
Many an Overlord has been defeated because they were unprepared for their enemies tactics. Let the whole world know you have only one gun, one power supply one control room, let it be common knowledge. Let the Hero plan, let his sidekick offer funny side comments and when they come for you let them be surprised to death. If the Death Star had had a second shield generator the rebellion would have been taken to pieces.
Redundancy and repetition are key ingredients to your reign. So que up production and make a few spares.