one problem that I have with this manga is that biologically some of the skills they use aren't possible no matter how hard you train, the human body has limits, you just can't go past it just from effort for example the double edge cut, you can't just swing once but cut twice normally and if you were to cut twice in the same amount of time it would take to cut once (a substitute to double edge cut) how fast would you have to swing?, how hard would you have to train? then he goes even further and does 3 cuts. but it's not really a big deal since it is a fantasy story.