Here's a tip, rush them with powerful attacks when they put those snake arms into the ground. Also it would be a good idea to cut off those arms if possible as they become their own entities in the yo-kai realm.

I find the best way to take care of him is an amrita-gauge Yokai Shift build. If you learn the nuances of YS (which I didn't for an entire playthrough), you can absolutely smash him by chaining soul core attacks into a Yokai grapple into more soul core attacks. It still wasn't easy, and a YS build is inherently volatile, since getting hit with the wrong move can nearly end the entire attempt, but I was able to kill him before the first dark realm phase ended this way.

Had a very unfortunate time with this guy, I killed him once, but he killed me at the same time & although the game counted the kill, it respawned me back at the shrine & I had to do it again, that wasn't fun

Japanese name is 夜刀神 which translates to something like God of the Nighl Blade or Night Blade God

This boss is easy enough in its main encounter, because its boss room fits its size. It is (not accounting for level scaling) way harder in the Calamity Pulse online mission, just because the close quarters of that boss room make for an absolute fustercluck when he goes into the Dark Realm phase of his fight. It can be remedied by weakening (not killing) its arms before it goes into the Dark Realm, so you can kill the snakes quickly, but if you can't kill them quickly they will divide your attention and make it very easy to get chained to death by their lighter attacks and its heavier attacks. The fact that you can't target lock the arms as well as the head and the body is a killer, especially as the arms are a higher priority target than the head because they facilitate half of its attacks in normal mode and (especially on higher difficulties) can be tanky enough to not go down fast enough in the Dark Realm.