This Trophy is for beating Asagi and getting her to join your party. A bill to open up her line of missions becomes available at the beginning of Episode 6. Once this is passed, you'll find a new mission below the Episode 6 missions.
There are two maps for this, one involving enemies around level 35 and then the actual fight with Asagi herself. Neither map is particularly difficult, just make sure you have the best available equipment on your team and that they're around the same level as your opponents.