Simply progress through story mode until you get to the level "The Well of Souls". It doesn't matter whether you enable cheats in Extras-Menu or not, as soon as you finish the level, you will get the achievement.
Just follow the video guide below, or use the following guide, which will also give you an idea where to find the hidden items in the level:
http://mycheats.1up.com/view/section/3165287/23521/lego_indi...Credit for the video goes to
http://www.gamerwalkthroughs.co.uk/