I don't know, it seems to me you are making an already overpowered villain even more powerful. As if it was a demon king in a sword & sorcery fantasy series. What's the point of making the devils based on a very particular thing if the plot doesn't stick to it? When the Gun Devil has the power to erase any city center, it already has a huge amount of influence to use to get things done indirectly. That's far more interesting than it being able to do everything by itself, in addition to the huge destructive power.