We have plenty of stories where MC has firearms to tip the scale in battle in a every idealism story where sword fight and magic are common. Let’s have some twist, what if the bad guys are technologically advanced and have like modern guns and vehicles and good guys have sword and magic? Interesting concept right? Yes but the problem is that most author would make hero too op and way too bland to be suspenseful. There is a manga called sweet nectar or something which written by same author who wrote seikon no quazar: you know breastfeed=power... That manga gives the bad guys more technophiles.
Demon king and hero story, the one with the same voice actors with spice and wold can't remember the title but the villain uses guns
One more thing. MC and good guys must not be very OP, just as I said above. This manga clairfies this: https://www.mangastream.cc/manga/shinju-no-nectar/ Excluding ecchi stuff, the world pretty much fufill my criteria, is there anything similar to that? Do they have modern weapons The villain must have modern weapon and MC and his harem will have trouble dealing with them and have to use brain and magic to overcome them. Villains er technophiles.