Recruited Inuou to get this chievo.
You have to complete "Supply line in the Desert" to unlock this mission.
This mission is called "The Strongest" http://translate.google.com/#en/ko/The%20Strongest%3F
You need to make sure Inuou don`t retreat. This is an easy mission and you should have no trouble taking care of this on. Inuou will join you after you have completed this mission