Unlike most of the character-specific achievements, this will unlock just fine in training mode. Pick whoever you want, and make the training dummy Kohaku (solo). Go into the settings and set her to "crouch."
Now wait a real long time-she has three little speeches she'll eventually go into while crouching, but this process can take several minutes if she decides to repeat herself. Fortunately, there's also an achievement for hanging out in training mode for fifteen consecutive minutes, so maybe knock these out together. I got this one after about eight minutes.