Way of the Samurai 3

This is pretty simple to do. Its called 'Samurai Slayer' and all it takes is completeing the game with an ending using instant kill mode. Instant kill mode can be obtained by earning a certain number of Samurai points. Not sure what that number was, but after unlocking this mode, I took the route of ending 11, killing Shozen in his room before the Nobunaga Inkling.

As zalm said the achievement is titled 'Samurai Slayer' and you have to obtain an ending (Leaving Amana does not count nor does death) for the achievement to unlock. To unlock instant kill you need a certain amount of samurai points, I believe it's 5,000. The ending I ended up doing was 21 considering the mob pretty much kills itself and I had no trouble with Shuzen or Zaji, however I think ending 11 would end up being a bit easier. Just use a weapon you are either comfortable with or a weapon with a long range attack so you always get the first hit. Good luck!