First Phase
During this phase the Eye will spend most of its time floating above you. It will summon Servant of of Cthulhu which fly towards you to attack, much like Demon Eye 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 Iron Broadsword to get rid of them quickly.

A good strategy in this phase is to aim mid-range 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 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 its first charge.

If you manage to dodge it during his charging attack, and he goes underground momentarily, hit it with fire arrows before he disappears in order to set it alight, and you can see it On Fire!on fire, making it visible. Keep in mind that in Desktop version , bosses have minimap icons to increase their visibility.

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

Eye of Cthulhu has 12 defense 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 no 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 this phase, Hermes Boots or a variant of them is an asset. With a wide enough arena, running in a straight line while the Eye of Cthulhu does his quick charge is fast enough to avoid any damage.

Expert and Master Mode
In the first phase of the fight it is more aggressive, acting very similarly to first phase it spawns servants while floating around the player, and will occasionally make a few lunges. It also enters the second phase at 65% health instead of 50%, spawning a lot of servants in that process. A piercing weapon is recommended for eliminating the servants.

In this part of the fight, after the 3 normal charges it will perform a brief series of extremely quick dashes at a steeper angle. As its health goes down, the Eye will use this ability more frequently, until its only action is to dart haphazardly back and forth, making it rather hard to hit. To avoid the charges, Hermes Boots should be used at the bare minimum.Starting the battle just after the sun sets (7:30 PM) will provide the maximum amount of time to defeat nocturnal bosses. In Hardmode, the Moon Charm and/or Moon Stone are also useful at night.
Prepare your Boss-fighting arena with adequate lighting, Campfire and Heart Lanterns for health regeneration, and Star in a Bottle for mana regeneration if you use magic. Bast Statue will provide a significant defensive boost. Garden Gnomes will reduce damage taken and increase damage dealt due to the luck. Honey pools can also be used for extra health regeneration.
Sunflower provide the Happy! buff to players within a 50-tile radius, which grants a +10% movement speed bonus and 17% reduction in enemy spawn rate.
Always carry at least one stack of the highest-tier healing potion available (Lesser Healing PotionLesser, Healing PotionNormal,Greater Healing Potion or Super Healing Potion Relying only on natural Health Regeneration is not a good idea.
The same goes for Lesser Mana PotionLesser, Mana PotionNormal, Greater Mana PotionGreater, or Super Mana PotionSuper Mana Potions if you use Magic. The Strange Brew could also be used.
It is suggested to build houses for the Dryad and the Nurse