Mission 8: Kanazawa City
In the middle of this hectic and war torn mission, there is 2 bamboo cages with prisoners that have been captured. The way I did it, was to get Yuki on the upper wall above the cages first, then use the door at the end of the wall after killing all enemy personnel first. Once you've done this, sneak down to the ground and keep a watchful eye on the Samurai nearest you who patrols back and forth. There is also another who patrols too. Before you do anything, take out Straw Hat guard and hide his body in the door you came through.
Now for the next bit which is tricky. I managed it so I know it's possible. Wait until the Samurai walks away from a cage, then open the door and hide again. Repeat for the other cages and your achievement will unlock. It's all about the timing with Yuki as she cannot melee kill a Samurai alone. But it's possible. Remember quick save is your friend here.