Recute Shimazwei fore this achievement
The mission you need to complete is called "A Former Key Figure" and is spelled something like this (http://translate.google.com/#en/ko/A%20Former%20Key%20Figure
) the translation is form Google translate so there might be some errors here.
This mission is playable after you have completed the mission "Supply Line in the Desert" and you have had the convention option a former key figure with Cassius.
The mission will end when you reach the red marked area right before Shimazwei with Begina .
Be aware that the grayed out enemies in this mission has the 5 hit gear on. Witch means you have to hit them 5 times before they take any dmg use light attacks (A) and teamwork to make this easy on you're self.