Guide:Eye of Cthulhu strategies

Terrain preparation
As with all bosses, preparing for the fight is most probably the biggest factor in your success.

When fighting the Eye of Cthulhu, a strategy I've always found to work is to make an arena composed of a border of dirt or stone with Wood Platforms placed every 6 blocks inside it so that you can evade the Eye by either running, jumping or dropping. Arenas with walls on either side can limit your horizontal movement, but will also provide a backdrop to catchShuriken and Arrows, so they can be easily pick up for reuse during the fight. Don't forget to place some lighting around the arena since you definitely do not want to be fighting in the dark.

Armor
Gold Armor is recommended for this fight, if possible. Even though the boss doesn't hit very hard, it constantly summons Servants of Cthulhu which do a surprising amount of damage and can be difficult for newer players to dodge.

Weapons
Because this boss summons many minions, weapons with a piercing attack are extremely useful. Shuriken are an effective weapon which can be bought from the Merchant. They will hit up to 4 times, allowing a well aimed shot to kill a Servant and hit the Eye twice - even thrice, if it falls back through the boss. A stack or two is more than enough to defeat this boss with relative ease.

Bows with Jester's or Unholy Arrows are good for their high damage. The former can be found in Chests Underground, while the latter can be crafted from Worm Teeth found as drops in The Corruption. Because they are piercing weapons, both arrow types can hit the Eye multiple times.

Throwing Knives act similarly to Shuriken, but they only hit twice. Even so, their comparable throwing rate and higher damage makes them a good compromise between Shuriken and bows. They can be found in Chests or bought from the Merchant during Blood Moons.

Grenades cause high damage and hurts everything in its blast, but may pose as much damage to the player as it does to the boss. Since the Eye will be spawning Servants or charging at the player most of the time, it's difficult to keep a safe distance from the explosion. If desired, Grenades can be purchased from the Demolitionist.

A Thorn Chakram is one of the most effective weapons, to the point where you can kill the Eye thrice in one night easily. It does more than double the Shuriken's damage and comes back to you, so you don't have to worry about running out. However, acquiring the materials for the Chakram may be harder than the boss itself due to the dangers of the Underground Jungle.

Because the boss typically flies out of reach, melee weapons are of limited use. No matter what the weapon of choice, weapons with higher damage per second are preferred. The greater your damage, the shorter the fight and the better your chances of survival.

Potions
Finally, bring a batch of Lesser Healing Potions at the very least, in case you start taking too much damage. Two other helpful potions are the Regeneration Potion and the Ironskin Potion. They are easy to brew and last a few minutes each, so you will probably only need a couple.

First Phase
During this phase the Eye will spend most of its time floating above you. It will summon Servants of Cthulhu which fly towards you to attack, much like Demon Eyes, but will fly through walls and the ground. The Eye will still hurt you, if you run into it, so do not disregard it while you kill the summons. If you start to get swarmed by too many Servants, temporarily switch to a fast swinging weapon such as a broadsword to get rid of them quickly.

A good strategy in this phase is to aim ranged weapons at the pupil of the Eye, since that is where it spawns the Servants. That way, you can kill the servant and damage the Eye. If you stand directly below the Eye and aim straight up with a piercing weapon, after a few seconds your projectile will fall back down and hit the Eye again. When it stops spawning Servants, the Eye will charge at you, attemping to ram you 3 times before hovering once more, so be prepared to evade. It turns rather slowly, giving you time to prepare your next dodge.

Rinse and repeat and the first phase should go down easily.

Second Phase
For the last half of the fight, the boss sheds its iris to reveal a fanged mouth. Make sure to take advantage of its transformation sequence as much as possible, since it just spins in place for a few moments and you can still damage it. It moves slightly faster and deals more damage when it hits you but it also has less defense. It will not be summoning anymore, so this time it will simply keep rushing at you. It'll charge you 3 times, and then pause for a few seconds during which you can attack freely. Try to hit it with your strongest weapon. For newer characters who haven't made it to the Gold/Silver stage, Shurikens are still your best bet.

Survival
Like the Eater of Worlds, the Eye of Cthulhu will flee if the player dies during the fight. It will also flee if the player fails to defeat it before early dawn (about 4:30am, if you're wearing a watch). The best way to avoid this is, like the other methods described above, a matter of preparation. Start the fight early after sundown, take lots of potions with you, and wear the best gear available to you.