Ah, the roof of Ashina Castle. The boss at the end there is sort of described as the brick wall that basically is the “final exam” that determines if you’re ready for the “real game” so-to-speak. I’d personally recommend doing that boss and Lady Butterfly around the same time, but you could do it in either order or avoid LB entirely if you wanted to. I usually reach both bosses before fighting either, then fight them back to back, that’s just the way that makes the most sense to me, but everyone’s different.