These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
That One Boss: Gregorio and the Sanbone Trio. In Master Mode, arguably every boss/stage is an example of That One Boss.
For those who don't see it coming, the first battle with Ben-K, which is 100% Guard Phase (read: flurry of notes from all directions), can be a particularly insidious showstopper. If they were going for the fact that you were meant to barely escape from the battle alive, Koei and iNiS did one hell of a good job.