The other solution is correct (apologies), though it is doable without set points in the skills listed.
This is unlocked in Quest 4 Gorgon. Continue on into the Portrait Gallery and into the back room with the Gorgon/Medusa Puzzle. Point Medusa at the Shield, The Shield at the Sword, the Lantern at the Shield, and the Sword at Medusa. This will put you inside the secret room with Emily. Once you finish talking to her (regardless of choices) you will search the room with her.
Pandora's Box is actually the vase/jar sitting on one of the corner pillars in the room and you need to translate the inscription using either the left or right most options (If you don't have points in those skills just use one of your Carmelite Water's with the D-Pad to get a freebie) Then on the next prompt choose to Open the "box" to unlock the Achievement.