The Eater of Worlds(EoW) is the most essential boss for new players to defeat. It drops shadow scales and tons of Demonite Ore so you can craft Shadow Armor and other Demonite weapons.This will tell you the weaknesses, strengths, and things to know about the EoW!
Strengths: Is huge, splits up when you hurt it in certain places, must be fought in The Corruption(monster distractions) needs to be summoned from Chasms.
Weaknesses: Is overall wea, slows down when hurt, is easy to dodge.
Arena: 2-3 rows of wooden platforms for dodging.
Potions: Lesser Healing, Hunter, Shine, Thorns.
Weapons: All you need are Shurikens and an Iron Broadsword(at least).
The Fight! Start on the top row of your arena and summon the boss, if it comes straight out of the ground then through a Shuriken down the length of it. When it splits up just stand up top and spam Shurikens when you see a part of it. When it is dead go down to collect you spoils of war!