This achievement is story related and once again fairly early on, it's rewarded after your first boss battle against 2 stone knights and you learn the ability to fly. The puzzle before the boss is simple enough, use Igniculus and the LT shine ability to light up the different colours to match the crystals to unlock the trapped lady.
The boss battle is easy if you have fought every enemy you have come across (and why wouldn't you have this early on) While the 2 Bosses may have striking abilities, healing abilities and can buff there own speed ect they should pose no threat as you beat them down. Just remember to slow them down with Igniculus throughout the battle and collect the MP and HP nods from the little bushes so you can keep using magic for a faster finish.
EDIT: This achievement may not be unlocking quite the number of people, so it may be a little glitchy. There is no news as of yet as to why this is.