This Achievement is for getting your Store Level to 15. It doesnt sound like much but it is such a grind to complete as it takes millions of Gald to go from 14 to 15. You store level is increased by spending Gald at the store whether it be buying, selling or enhancing equipment The best way to do this is to work on it along side the "A Veritable Gald Mine", the Titles for enhancing weapons and Magilou's Title for buying equipment.