Watch out, this achievement is MISSABLE and can be tricky. To unlock it, you need to beat the shell mini game in the third chapter (Corte Vecchia) on your first try (the game with the 3 colored marbles). Note that each time you attempt this puzzle, the pattern will be different from your previous attempt. The only tips I can give you are don't blink and restart the chapter if you miss one marble. Good luck !
The Shell Mini Game (Eagle Eyed Achievement):Walkthrough of Corte Vecchia (Third Chapter):Full walkthrough of the game: