Use a rosary or glaive for this battle. Rosaries are convenient since they’re ranged, while glaives can deal more damage one you get a chance to strike. Just keep hitting its arms while dodging its attacks, and one they’ve all turned red, start cutting them off. I think that's it... it can be a frustrating battle for sure. 

I think it's just a giant starfish monster. In Japanese it's called "starfish demon" (海星鬼). The only particularly special thing I can think to say about it is that its scales resemble a traditional Japanese wave design based on concentric circles.