The Eater of Worlds can be a very tough boss if you're not prepared. A good strategy for tacking this enemy is to take the time to build a large enclosed arena a few 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, because they will be able to deal the most consistent ranged damage.