This event occurs in the town of Devour at the beginning of disc 3.
After visiting Kluke at the Inn after she passes out, head to the cliffs and Shu will break down and try and summon his shadow. Make sure and save your game before heading to the cliffs in case you fail the scene.
This event will consist of 3 separate segments of mashing the "A" button.
The good thing is that it's only one button, but the bad thing is that you have to be extremely fast. If you were successful the achievement will pop up at the beginning of the proceeding battle.