The Eater of Worlds (or EoW) is one of the bosses in Terraria. It is found in The Corruption, and summoned by either breaking three Shadow Orbs or by using Worm Food. Preparation for this boss is both extremely helpful in the fight, and is relatively simple to complete. The following are some hints, tips, and strategies to help you defeat it.
Terrain preparation
Wood Platforms help with dodging attacks.
Like all Bosses, preparing an arena for the fight is probably the biggest factor in your success.
Standing on a wide, elevated platform helps tremendously, because you will be able to see him approaching. The platform can be further upgraded with several levels of Wood Platforms, to further increase the player's mobility.
Since Worms cannot navigate in the air, this setup will drastically reduce the Eater of Worlds' maneuverability. In such a scenario, the Eater of Worlds can almost always be evaded; getting hit is often a result of the player's own risk-taking to deal extra damage. Currently, the Eater of Worlds can only be spawned and fought in the Corruption. As such, it would be best to create an artificial corruption area to fight in. Alternatively, you can find naturally occurring Corruption and create an "arena" there. However, if you plan to create a floating arena, you may have to hollow out a large section of the Corruption to do it, as going too far above the Corruption will be considered exiting it, causing the Eater of Worlds to despawn.
If you are summoning Eater of Worlds by breaking Shadow Orbs, building a full-scale arena for every third orb or even trying to get all the way up to the surface is probably impractical. In that case, you will still benefit from taking time for terrain prep as shadow orbs are located in confined spaces and not being able to move around can get you trapped as Eater of Worlds circles around you. Bring your preferred tools for dealing with Ebonstone (Purification powder with a pickaxe and/or Bombs or Dynamite if its your first fight, and a Nightmare Pickaxe or better thereafter) and clear out any stone that impedes your movement. Give yourself a clear escape route out from where the Shadow Orb is, ideally to an open area with a good amount of horizontal clearance (there tends to be at least one intersecting the chasm above the shadow orb; if not, you should probably break out the Sticky Bombs and Dynamite to make one). Place Wood Platforms over deep water and any chasms that present a falling hazard. Smooth out large protrusions on the floor that will get in your way, especially if you're using Spiky Balls. In general, horizontal freedom of movement will likely be more important than vertical, especially as Eater of Worlds will be attacking you either through the ground or the ceiling. Light everything up well, and clear out any stray monsters from the area before breaking the orb. A Grappling Hook or Ropes are extremely helpful equipment in this scenario, and any items that well help you ascend up to your prepared arena quickly, such as Rocket Boots, are good to have.
Gearing up
Armor and Accessories
Gold armor, Meteor armor (with the Space gun), or Platinum armor is recommended but not necessarily needed, as it is not very hard to dodge the Eater of Worlds' attacks. It is possible to kill the Eater of Worlds with just Iron armor or Lead armor, but the fight will be a bit more dangerous. Pumpkin armor is another viable option, rewarding slightly less defense than iron, but increasing your damage output, which can ensure the fight is completed quickly. Rocket Boots are recommended as they can help dodging. Accessories with Menacing or Warding will be most efficient here, but are not required.
Weapons
Because of its large size and multiple body parts, weapons that can deal damage to multiple sections should be used. The Vilethorn will not deal damage as quickly as other methods, but will have a balanced damage distribution throughout the entire body. The Vilethorn is obtained from Shadow Orbs in the Corruption. Furthermore, this is a relatively safe weapon to use against the worm as it can be deployed in the worms path and will remain for a short time doing damage while giving you time to dodge. Upon destroying the Eater's head, it will pause briefly while the body segment behind the head is converted into a head; aiming the Vilethorn directly at the head can destroy over a dozen segments in quick succession in this way. A similar tactic can be deployed using Water Bolt if you were lucky enough to find one in the entrance of the dungeon. In confined spaces, the water bolts can ricochet many times and deal tremendous amounts of damage. Your mana should be kept topped up by the stars dropped from killing the segments. Furthermore, staffs (preferably a Ruby or Diamond Staff) can also pierce multiple parts and deal great damage, while using little mana. You can also use spiky balls, which will sit there as the Eater of Worlds comes to you, and he will keep touching the spiky balls, so he will constantly take damage.
Guns can be very effective at killing worms, particularly if Meteor Shot is used to pierce multiple segments. The Space Gun acts similarly, able to hit two segments in one shot. Bows don't have as fast a rate of fire, but by farming Devourers for their teeth and upgrading to Unholy Arrows (you can also farm Eye of Cthulhu, as it drops them straight), the piercing effect makes them a great choice. Jester's Arrows work just as well, if not better, and are fairly easy to get. The Demon Bow is also relatively easy to get by this point and will work well. If a shot can be lined up, about 30 damage can be done to 10 segments, finishing the fight very fast if great care is taken when aiming. The Star Cannon, while very expensive, yet still obtainable for your first encounter, makes a mockery of it, ripping through multiple segments and destroying virtually any in one hit (you will need to get lots of Fallen Stars for this). Also, a Bee Gun can be very effective against the eater of worlds, because of it's fast-rate homing projectiles. Even before entering hardmode, it is easy to farm the EoW using a minishark and stacks of musket balls. As it is relatively simple to make a Vile Mushroom and Rotten Chunk farms, you can stock up on Worm Food and summon it continuously and exploit the lesser health of its head to quickly kill it with a Minishark or any rapid firing gun.
Non-explosive consumables, such as Shuriken and Throwing Knives, can be effective since their piercing attack will hit multiple segments while allowing the player to keep a safe distance. Explosive consumables, like Sticky Bombs and Dynamites are very effective, killing all pieces caught in the blast. The player should be careful, however, to not get caught in the blast themselves and should keep in mind that this will destroy the arena. Grenades and Beenades are a less destructive alternative.
Spiky Balls are especially useful, as they can be thrown down where the Eater of Worlds is likely to surface, in the same manner as Vile Powder. This can further be combined with a Flail, firing a Harpoon straight down, or firing a Water Bolt in a confined space, while using a Grappling Hook to keep you from moving. When the EoW surfaces it will quickly be destroyed, although this strategy may require a reasonable amount of armor and health. (You can acquire the Water Bolt without killing Skeletron if you can find one without having to go down into the dungeon.) Similarly, Molotov Cocktails are extremely effective against the EoW, leaving behind multiple flames on the ground that do ~40 damage each and hit every segment of the EoW that moves through them.
Alternately you can lure the EoW in attacking you horizontally while firing a Harpoon at its head. The high firing rate of the harpoon will allow you to kill it without even being hit and this will prevent the EoW from splitting into other worms. Furthermore the head (with a much smaller health and defense) is easier to kill, turning the next body segment into an equally painless target to attack.
Large swords like the Phaseblade, or the War Axe of the Night can deal damage evenly if swung repeatedly at the long worm's arcing body. By focusing attacks on the head, the next segment will already be weakened, likely falling in just one more hit. Because of the way the worm acts when its head is destroyed, it will quite literally feed itself into the meat grinder. The Chain Knife is also a great weapon to kill the boss, but takes a while to kill it. If you have a Ball O' Hurt from breaking Shadow Orbs, or a different flail, you can easily swing it into the EoW and inflict massive damage to many of its segments. This method requires more defense and/or health, however, and preferably a way to avoid knockback.
If you are lucky enough to have found an Enchanted Sword Shrine and obtained an Arkhalis, you may find they are very effective against the EoW.
The choice of the weapon will essentially boil down to how safe you want play, or how fast you want to kill the Eater of Worlds.
Potions
Healing Potions are recommended, just in case you slip up and do manage to get hit one too many times by the Eater of Worlds. They do not have to be in the hotbar as they are easily accessed using the hotkey for healing (default is Quick Heal). Ironskin and Regeneration Potions can really help mitigate any hits you take. Hunter Potions can be very useful in spotting the Eater of Worlds as it burrows, making it easier to fight even without a raised platform. Last of all a Bowl of Soup, Cooked Marshmallow, Pumpkin Pie orCooked Fish will also give you a small additional buff.
The Thorns Potion will damage the Eater of Worlds every time it hurts you and is easy to craft.
An easy way to get out of the chasm if you wish to fight on the surface is to use a Gravitation Potion and a Lucky Horseshoe. A grappling hook like an Ivy Whip can also be very useful.
Managing the Worms
Killing a segment of the Eater of Worlds will split it at that point into two worms, with a new Head and Tail forming on either side of the destroyed segment. Of course, the more worms there are, the harder it is to dodge their attacks. A fundamental strategy when killing the Eater of Worlds is correct management of each worm. Avoid splitting worms when you can. If there are little worms, concentrate on killing those first, before returning to the main worm. If a worm has 3 segments or fewer, aim for the middle. Otherwise, aim for the head and/or the segment behind it. Standing in a small box with a weapon that bounces off walls like the waterbolt can deal lots of damage, but the Worm will do lots of damage, so you can rig a heart statue in the box for greater health.
Survival
Dying during a Worm fight will result in respawn at your spawn point. When you die, if your spawn point is far from the place you've been killed, the Eater of Worlds will retreat, and you will not be able to fight him unless you summon him again. If your spawn point is nearby, the boss will be roaming around your death place. When summoned again, the boss will be back at full health, so caution is advised when fighting. Health potions are also recommended as these will increase your lifespan, allowing more time to kill the Eater of Worlds.
Farming
The EoW can be a good source of money if you sell the Demonite Ore and Shadow Scales that it drops. With endgame armor and weapons, you can quickly defeat this easy-to-summon boss.
Hardmode weapons that swing continuously such as the Phasesaber and Excalibur are extremely effective against the Eater of Worlds. Spears and higher tier Flails can also quickly dispose of the EoW. Simply grapple yourself to the ground (or use a Cobalt Shield) and keep attacking/keep your flail below you by holding ⚒ Use / Attack. If you have Molten Armor, you will be able to take any damage the Eater of Worlds dishes out, as its killed segments tend to drop Hearts, which will heal you.
Expert Mode
Like all other bosses, the Eater of Worlds is much harder in Expert Mode. When playing in expert mode, EoW will shoot large amounts of Vile Spit when above ground. This, along with expert mode's increased health and damage, makes the fight extremely difficult above ground, so you may want to try fighting underground.
If you do choose to fight above ground, then the best thing to do is always move - vile spits have a fixed trajectory, so most of them will miss if you move constantly. Since moving makes hitting the boss harder, Meteor armor + Space Gun is a good gear combination since it allows you to shoot freely beneath you, hurting the worm while you don't take damage. Making an arena to fight the EoW is highly recommended to keep the Eater of Souls away from you while fighting the boss.
If you managed to get a Reaver Shark, using Molten Armor and Molten Fury with Hellfire Arrows can let you defeat the Eater very easily while also getting a lot of protection thanks to the armor.
See above for weapon and arena recommendations.
