Hey! Hitting the head is instant kill, so of course it can't be that easy. In previous versions it was easier, but with v5 you have to first cut the legs of the guards to rock their balance. Then the head becomes vulnerable, with an icon indicating this.
You are right that the game doesn't inform this properly to newcomers. I still need to figure out how to give players tips about this new weakness system. Sorry if it felt too unfair. I know the game can be a little bit more difficult at places because of this new system, I'm still looking for better balance.