You will need to get an item to Level 15 for this achievement. First, you will need to get an item to Level 10. Please see "The Best Items" achievement for how to get an item to Level 10. Next, you will need to get 5 more of the item you want to get to Level 15. I highly recommend using an item you can commonly get for this achievement since it will be easier to get more of the item. The 5 items do not have to be leveled up at all; they can be kept at level 1. When you have 5 more of the item as well as a sufficient amount of Happy Stars to spend on leveling up the item, choose the item you want to get to Level 15, pick one of the other copies of that item as a material, and then pick the other materials. Repeat that four more times to get the item to Level 15.