1. make a small world
2. downloand terraria world viewer to get a map of your world
3. explore that program a little. you will find a thingy that allows you to chouse the item you want to find
4. take every other object exept the item you want to get
(5. if the item is in that big dungeon whit that skelectron boss get in, get the item, use magic mirror, tadaa)
if its not
(5.if the item is at under ground get some good armors so you can fight agains those scary worms, dark slimes and skeletons.
6. use the map thingy and go above the object
7. dig downwords as long as you get to the treasure chest
8. grab the item and use the magic mirror, if you dont have it get your self killed (you will lose some money but you will get your thing))
if its at floating island take some dirt blocks whit you
jump and put dirtblock under you and continue as long as you hit the island (you will find them by using the map thingy