Cosmic Observer Strategies

Cosmic Observer can be quite difficult for players who don't have wings or any other mobility source yet.

Spawn Condition
Cosmic Observer has a chance to spawn naturally in Space during Hardmode. Using increases the chance of this boss' spawn.

The Fight
Cosmic Observer usually stays on one side of the player, changing sides by charging towards the player. It occasionally charges up power and shoots two Observer Fireballs, which explode upon contact with either a solid tile or the player. It also stands still and shoots an arc of eight Observer Shards at the player. It uses its attacks more often and moves faster as it loses health, and reveals its core visually. When low health, it sometimes spins, releasing a barrage of Observer Shards around itself. In Awakened Mode, Cosmic Observer has a new attack which charges up power and shoots a laser. At first, it releases the image of the laser which sticks to the player. If not beaten fast enough, it will use the laser as an attack and deal enormous damage. This attack is almost guaranteed to kill a player at full HP.

Terrain Preparations
The arena for Cosmic Observer can be done in two ways: Either you can make a simple arena at Space with 3-4 rows of at least 50 platforms on each row. Or, you can make a skybridge and try outrunning it as well. If the boss had spawned by surprise and had no preparations in Space, it can also be lured to the ground and you can fight him in your ground arena as well. Aside from the arena's length and style, having Campfires and Heart Lanterns are a must, as the attacks of this boss deal high damage and they might help you recover quickly. Also having a buff station for a specific class like Sharpening Station for Melee are really useful and Star in a Bottle for mages along with Heart Lanterns work well.

Armor
The armor for Cosmic Observer depends on your farming and progression. Generally, you can use any Hardmode Armor starting from Cobalt Armor or Palladium Armor but if you want less intensity and more survivability, either Adamantite Armor or Titanium Armor are the best options. Also, you can use Frost Armor (if you are melee/ranged) or Forbidden Armor (if you are mage/summoner) if you had farmed for Frost Cores or Forbidden Fragments. Spider Armor is required for summoners.

Weapons:
For Melee Users: For Melee class, any sword that has a projectile is useful. These weapons include such as Beam Sword, Frostbrand and Manashard Longsword. Also Uranus and Amarok/Hel-Fire Yoyos work quite well with Yoyo Bag as they can deal moderate DPS against the boss. Nihongo is also a great weapon against this boss as it shoots three projectiles and the spear itself deals great damage as well. If you had built a lot of defense and damage reduction, you can facetank the boss using Fetid Baghnakhs as well.

For Ranged Users: For Ranger Class, Daedalus Stormbow is one of the best options with Holy Arrows or Hellfire Arrows. Any Hardmode Ore Repeater also gets the job done. As a gun option, Clockwork Assault Rifle works well with three bullets shot and one bullet consumed. Also Onyx Blaster is another great weapon that can deal high DPS. Having Crystal Bullets or Explosive Bullets can deal great DPS to the boss. Star Cannon is another powerful weapon option if you have decent amount of stars to spend.

For Magic Users: For Mage Class, there are a couple of decent options you can have: Meteor Staff deals great damage to single targets, but its high mana cost can be a small nuisance. Also Poison Staff, Frost Staff, Nimbus Rod, Flower of Frost work great against the boss. Laser Rifle is a decent choice too. It has quite moderate mana cost and damage. As a spellbook option, Crystal Storm and Golden Shower/Cursed Flames are great as they deal great DPS and inflict debuffs. Frost Mine deals high damage, but has high mana cost. And aiming the shots might be difficult. Hardmode Ore Wands also work well as they have unique attacks that have decent damage and decent mana cost to spar. Crystal Serpent and Sky Fracture are also great damage dealing weapons if you have patience during farming to get them.

For Summoning Users: For Summoner Class, Spider Staff or Pirate Staff are the best options you can get at the current point. Fire Harpy Staff also works decently as it is the best for Pre Hardmode. Spider Queen Staff along with any of the OOA sentries can be used as backup damage dealers.

For Throwing Users: For Thrower Class, there aren't much options and as armor, it won't help much as well. But there are a couple of options obtainable in Hardmode: Dance of Darkness and Shade of Light are decent weapons with great damage output. Manashard Dagger also works well as shoots multiple daggers. As consumables, Beenades are one of the best in early Hardmode as well. Bone Glove with a decent amount of bones, Bone Javelins and Molotov Cocktails are also great choices.

Accessories:
In general, having Fire Treads is a must as it grants decent movement speed and flight. Also either Angel or Demon Wings work wonders. Any Horseshoe Balloon or Bundle of Balloons also work great. Any emblem that Wall of Flesh drops is a must, if you are playing a specific class. Chaos Flame Flask and Great Thunder Idol are great choices, as they grant offensive stat boosts decently and the flask grants fire damage on attack, which this boss isn't immune to. Defensive supportive accessories like Charm of Myths, Star Veil and Frozen Turtle Shell will increase your survivablilty. For mage, Mage's Focus and Shield of Magic work really well. They grant many boosts to mages when worn together. Illusive Charm is also another decent choice. Any Shield item you have at that moment is really useful. Either Inferno Bulwark or Ankh Shield work really well. Brand of the Inferno is another decent accessory that grants fire damage and %10 damage boost.

Potions:
You will surely need Healing Potions. If you had killed biome Mimics a lot, Greater Healing Potions work well too. For universal buffs, having Ironskin, Swiftness, Regeneration, Endurance and Heartreach are a must. Wrath and Rage potions are great for damage, Lifeforce is good for more survivability. For Ranged, Archery and Ammo Reservation potions are required. For Mage, Magic Power and Mana Regeneration potions work the best. Also having Mana Potions or Greater Mana Potions, even Super Mana Potions if possible are great to have. For Summoner, Summoning Potions are needed for more minions.