The Eye of Cthulhu is usually the first boss a player will encounter. It has a one in three chance of spawning once a player has met a few base conditions: Having at least 200 health, having at least 10 defense, having three NPCs, and assuming that the player hasn't already defeated the Eye. Since it is so likely to appear preparation is almost vital, especially for newer players. The following are some hints, tips, and strategies to help you defeat it. Note: if you acquire a Suspicious Looking Eye and use it while the screen says, " You feel an evil presence watching you" it is possible to fight two Eyes of Cthulhu at once.
First option for battle
Gather up potions and have at least a gold broadsword for a sword and a golden bow. At least full gold armor is required for this strategy. Silver or tungsten is fine too, but gold is recommended.
Terrain preparation
Fighting arena
Since this is the first boss you will face, you are limited with items to put in your arena, but campfires would boost your life regeneration. As for the terrain itself, rows of wood platforms will be the best choice. This gives you maximum mobility, especially if you have a cloud in a bottle. Lastly put a nurse close to the arena for some quick healing if you need it.
Gearing up
Armor
The best armor to have is gold or platinum armor, but it's not required. Silver or tungsten armor is the easiest to get at this point, and it is a good choice for this fight. Iron or lead armor can be used, but the caution must be taken, especially in the second phase, and should only be used if you are a good dodger. Copper, tin, and wood armor don't have enough defense to take the hits of the 2nd phase and should not be used unless you have built a good arena and are a skilful dodger.
Accessories
To help in avoiding the second phase, using some form of jumping aid would definitely be useful, but is not essential. Hermes Boots help by allowing a chance to dodge the second phase's rams. A Cloud in a Bottle or similar accessory can also aid in avoiding the rams. If you find one, a Band of Regeneration can help speed up life regen, making lasting a longer fight easier. Panic Necklace is a useful accessory if you have a hard time dodging attacks, and the increase in speed after being hit can make being hit immediately after less likely. A Shackle with a Warding modifier can serve as a good accessory if there is an open accessory slot, and the extra five defense can be used to make up for lower-tier armor, like Iron or Lead. Also, in the mobile version, the Boots of Ostara, which are dropped by Lepus, help a lot. dynamite would help when in second form because it blows up everything so when it charges... dynamite!!!
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 three times, if it falls back through the boss. 500 or so are more than enough to defeat this boss with relative ease.
A Thorn Chakram is quite effective if you can obtain it before the fight. It can be thrown relatively quickly and it's unique ability to bounce off walls up to 4 times can kill both the boss and it's minions quickly. Also,the poisoned debuff from hitting the eye (occasionally) can sap it's hp making the fight slightly easier.
A Vilethorn is always handy, especially when the Eye is inside of blocks, as it deals constant damage and can be fired through blocks. Other great magic weapons are staffs (preferably a Emerald StaffRuby or Diamond Staff), which cause more damage while using less mana.
Bows with Jester's or Unholy Arrows are good for their high damage. The former can be found in Chests Underground or crafted with Fallen Stars, 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. However, one can also use Flaming Arrows, because the Eye of Cthulhu is weak to fire arrows, and takes more damage from them than it does from Jester's Arrows.
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. If the player is willing to run through the surface of the corruption, they can obtain Vile Mushrooms, to produce Vile Powder, combining the powder with the throwing knives for Poisoned Knives. In the first form, the Eye has a respectable 12 defense, which negates 6 damage from all attacks. This can mean whilst Shuriken are hitting for 4 on average, Poisoned Knives will deal an average of 7. Combined with their damage-over-time poison effect, this makes them more than twice as effective.
The Musket also works, but you need many rounds, about 200. But, if you obtained a Minishark, it is a very effective weapon, as it can kill the eye in about a minute, with at least 400 rounds.
Grenades cause high damage and hurt everything in their blast, but may pose as much damage to the player as they do 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 good strategy for Grenades is to lay them like a quick trap. As the Eye charges at you, you can predict its movements. Throw a grenade underneath you and run away as the Eye begins to charge. If timed correctly, the grenade will explode on contact with the Eye and you will be unharmed.
Because the boss typically flies out of reach, melee weapons are of limited use. However, the Starfury could fare well, as the stars falling from swinging it could deal high damage to the Eye. Even so, 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. (Regular Healing Potions can be used if one has access to a Glowing Mushroom farm.) Three other helpful potions are the Regeneration Potion, the Ironskin Potion, and the Thorns Potion. Swiftness Potion, Hunter Potion, Archery Potion and Bowl of Soup/Cooked Marshmallow/Pumpkin Pie wouldn't hurt as well. 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 Servants. 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 servants 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. After spawning 3 or 4 Servants, the Eye will charge at you, attempting to ram you 3 times before hovering once more, so be prepared to evade. It turns rather slowly and roars as it charges, giving you time to prepare your next dodge. It can be helpful to begin to evade after the 3rd servant is spawned, as this will often allow you to dodge his first charge.
Rinse and repeat and the first phase should go down easily.
Eye of Cthulhu has 12 armor during this form, which negates 6 damage from every attack. As such, higher damage, lower speed weapons are more effective during the first form, such as the Musket or a Bow.
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 any more servants, so this time it will simply keep rushing at you. When it does so, do not try to attack it- concentrate on dodging. 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 (exactly 4:30am, which you can determine with a Grandfather Clock or watch). The best way to avoid this is, like the other methods described above, a matter of preparation. Start the fight early after sunset (7:30pm), take lots of potions with you, and wear the best gear available to you. Normally however, the eye can be killed with shruikens in a little less than half the night, so while it is important to start the fight early, it is not essential to start the fight the moment that night begins.
Redux
As you upgrade equipment, the Eye of Cthulhu becomes easier and easier to defeat and is a great source of money. Craft stacks of Suspicious Looking Eye and immediately re-summon the Eye after defeating the previous one (keeping in mind that dawn is at 4:30 am). If you manage to dodge him during his charging attack, and he goes underground momentarily, hit him with with fire arrows before he disappears in order to set him alight, and you can see the On Fire! debuff animation, making him visible. If you have a spear or vilethorn, try to hit him while he attempts to hover above you again.
