This achievment can be unlocked on the third level "
Imai Town".
Once you gain access to the officials estate about half way through the level face the front door of the main building in the centre on the left hand roof close to the front path you will find a tutorial and a set of icicles.
Wait for the patroling samurai with the key symbol above his head he will stop under the icicles at this point interact with them and they will fall down killing the samurai and the achievement will pop.