I actually got him with Freya! Just don't jump when he does the thing and makes you think about it.

So the breather boss is out of the way. So far the really challenging ones were Sephex and Wolf Kanno. But I noticed that when you change around your setups, they don't get just slightly easier - you just win. In that way, it's pretty much the same type of difficulty as SMT or Dark Souls - you go in, die, take notes, and try again. It's fun.