This is rewarded for solving all puzzles in the game. You
cannot unlock this achievement, if you have skipped some of the minigames (sometimes you can do that by pressing RT)!! So it is very important, that you solve everything "on your own".
Again, I can recommend the guide by
MaGtRo, which is excellent:
http://www.gameboomers.com/wtcheats/pcSs/SH6/SH6.htmThe achievement pops right after solving the last puzzle (you don't have to finish the game at this point anymore).
Note: As stated and confirmed by
ugli, playing the missed / auto completed puzzle doesn't help. You have to get this in a single playthrough.
Reminder:There are many people who confirmed this achievement to be buggy. As for now, there is no known solution on how to unlock it, if you failed to. My only recommendation in this situation: Though a lot of work: Play through the game again.