Eternal Sonata

SOME SPOILERS.

You get this achievement by collecting Clave's 7 Soul Shards, all located in the hidden dungeon Mysterious Unison. To get to this dungeon, you first have to beat Ruined Body, the boss in the basement of the Double Reed Tower near the end of the game (part of the story). This will get you the Hero's Crest.

How go back to the Noise Dunes, just north of the Elegy City of the Moon. Then you have to travel either right three times or left three times. (It will look like you're moving in a circle, but keep doing it!) This will take you to the door of the dungeon.

The dungeon itself contains the toughest enemies in the game. It consists of 13 floors. Most of the floors are mazes of sorts, with a teleporter that takes you to the previous floor, and one that takes you to the next floor. Each floor (except 4 and 9, more on this later) acts as sort of a checkpoint. You can use the down teleporter to return to the entrance of the dungeon, and from the entrance you can immediately skip to any floor you've previously visited.

Locations of the Soul Shards:
1. Chest on the 1st floor
2. Boss on the 2nd floor
3. Chest on the 3rd floor (Floors 3-4 are interconnected, more on this later)
4. Buy from the spirit on the 6th floor. It costs 99,999,999 gold. If you need more gold (and you probably will), you can wait until the 11th/12th floors, where the monsters drop 3 million gold each.
5. Chest on the 8th floor (Floors 8-9 are interconnected, more on this later)
6. Rondo on the 11th floor

After getting these six pieces, you're allowed to finish the dungeon:
7. Annihilator on the 13th floor

About Floors 3-4 and 8-9:
These floors are interconnected, and have multiple teleporters that lead to different sections of the other floor. Essentially, each collective acts as one giant floor. 8-9 in particular is quite complicated, and you'll probably need to make or look up some sort of a map in order to get through it. Which leads me to this:

If you need detailed descriptions of the locations of the Soul Shard chests, a very nice user on GameFAQs named ElevenFour has made some great maps. They show everything on each floor, and they're particularly helpful with Floors 3-4 and 8-9. They should help much more than a text description or even a Youtube video.
http://www.gamefaqs.com/console/xbox360/game/934162.html

The achievement will unlock after the 7th Soul Shard is collected. This achievement can be done on any playthrough, although it is probably easiest your first time through.