If you have a friend with Xbox Live do the following to duplicate items in multiplayer. Join a friend's game and place the items you wish to duplicate in a chest. Once done, quit out to the dashboard without saving. Restart the game and return to your friend's world. Retrieve the items from the chest and you'll find that you now have an extra set. Save your game and return to the main menu, then repeat the process if you want.
The Eater of Worlds is a difficult boss if you're not prepared. A good strategy for taking it on is to build a large enclosed arena a couple dozen blocks above wherever you plan to summon the boss from. This will keep smaller enemies out and the gap will give you enough of a warning to allow you to avoid most of the boss's attacks. Bring a lot of Shurikens if you don't have any flail weapons yet, as they will deal the most consistent ranged damage.
Every night time stars fall from the sky. Collecting ten of these increases your maximum mana. On normal terrain this can take a long time and can be a risky in pitch-darkness. By creating a Sky Bridge (by making a ladder around a hundred or so blocks in the air and laying down stone or dirt in a straight line), you can create a long stretch of land for the stars to land on. With a speed-increasing item such as Hermes Boots or Necro Armor equipped, you can travel the length of your bridge while collecting stars very easily. Zombies will still spawn on the bridge and you might get the occasional harpy or demon eye, but you won't have to deal with rough terrain, chasms, water or anything that spawns from the corruption.
Each randomly generated world comes with at least one floating island, which contain a chest with unique loot and are interlaced with gold and silver ore. Finding a floating island is hard as the altitude depends on the size of the world you created, but building a "sky bridge" can help. On small worlds, building the bridge at about 200 blocks above sea level will put you within sight of an island. Keep an eye out for vines floating in the middle of the sky, as that's the bottom of your island.