Resonance of Fate

This achievement is obtained by completing Chapter 11. This one is a little tougher, as the story mission involves a solo fight using Zephyr only. From the start of the chapter you cannot reenter the safe house to save, but you can access the arena before attempting the boss fight, if you need to level up some more first. Once you defeat the boss, you get the rest of your party back, and can use them for side missions if you don't choose to advance right away.