OK, expanding the idea as I'm working on it:
The "[something] Mazoku" (need a good term, maybe something like "Tetsu" - "Iron", but I guess something more like "Tough Leather/skin"? Well, "Tough Leather" is "Jōbuna kawa", dunno if that could be reduced to "Jobukawa" or "Jobu"... an alternative is "Tekitou", which doesn't has a direct translation to English but can be used in a positive or negative way, and when used negatively it means roughly something like "vague" or even "half-assed") are a variant of demon that has appeared around the world, origin is Japan, were accidentally summoned by some jackass who wanted to use them as sparring practice without knowing how BAD an idea that was, and which have an unusual weakness... guns. You can kill them real easy with guns.
Which sounds very dang weird until you remember that Japan had one of the most strict firearms laws in the world pre-Vanishing (and even post-Vanishing it gets real iffy when it comes to allowing civilians to carry, plus the whole damn lot of Fiction stuff over there that is more devastating so better options are available) and you can guess that these monsters are a problem.