Basic set-up for bosses and how to summon them

Basic set-ups and general tips

Although there are many bosses in Terraria, each with their own unique attacks and loot, there are some general strategies to help maximize the chance at beating them.

One simple strategy that can be used is to use potions and other buff items. Potions can help give additional defense, damage, and other effects.

Another strategy that can be used is building an arena in the place where the boss will be fought. The arena should be optimized to give maximum mobility and space. A typical arena will use platforms to give the player additional movement options, but other things could be used, such as teleporters. The arena could also include objects such as the campfire and other related objects to gain additional buffs.

With most bosses, a player either has to run away from the boss and/or "circle" the boss.

In both animations, the green square represents the player, and the red square represents the boss. The animation on the left shows the "circling" technique, while the animation on the right shows the run away technique.

The Bosses and how to summon them

There are many bosses that appear throughout Terraria with various ways to summon them. These bosses have additional changes when the world is in expert mode. These changes include additional attacks, and additional loot. The table below summarizes how the bosses are summoned and when they can be first summoned. Note: this table doesn't include event bosses and mini-bosses.

Terraria Bosses
Boss name When it appears How to Summon Basic attacks Expert-mode changes
King Slime Pre-hardmode King Slime can be summoned by using a Slime Crown. King Slime also is summoned once enough slimes have been killed during a Slime Rain event. There's also a random chance of King Slime spawning in the outer edges of the map. King Slime acts like a normal slime, jumping towards the player, except it can teleport to the player if the player is too far away. As King Slime takes damage, it spawns blue slime enemies. In expert mode, King Slime will spawn in Spiked Slimes in addition to normal slimes. These slimes can shoot spikes at the player. King slime will drop Royal Gel in expert mode.
Eye of Cthulhu Pre-hardmode The Eye of Cthulhu can be summoned using a Suspicious Looking Eye. The Eye of Cthulhu can randomly spawn during the beginning of night if the player has at least 200 HP, at least 3 town NPCs, and at least 10 defense. It will stop randomly spawning once it is defeated. The Eye of Cthulhu alternates between spawning Servants of Cthulhu and charging at the player for its first phase. Once it reaches about half its health, it will start its second phase where it will only charge at the player. In expert mode, it will summon a few Servants of Cthulhu as it changes to its second phase, and the charges at the player during the second phase will become faster and more aggressive as it loses health. The Eye of Cthulhu will drop the Shield of Cthulhu in expert mode. Also, if a player is in hardcore difficulty, the 0x33's Aviators will drop.
Eater of Worlds Pre-hardmode The Eater of Worlds can be summoned using Worm Food or by breaking 3 Shadow Orbs in a world containing the corruption biome. The Eater of Worlds acts like a worm enemy, able to pass through blocks, with multiple sections. It is capable of splitting into multiple sections, and all sections must be defeated. It expert mode, the Eater of Worlds shoots Vile Spit at the player. The Eater of Worlds drops a Worm Scarf in expert mode.
Brain of Cthulhu Pre-hardmode The Brain of Cthulhu can be summoned by using a Bloody Spine or by breaking 3 Crimson Hearts. The Brain of Cthulhu will be invincible and have Creepers that float around it, which it uses to attack the player. It will teleport around occasionally. Once all of the Creepers have been defeated, the Brain of Cthulhu will enter its second phase, become vulnerable, and begin to try to ram the player. It will teleport more and move faster in its second phase. In expert mode, the Brain of Cthulhu creates false copies of itself. These false copies don't take damage from the player, and they don't do damage to the player. They are just a distraction. The copies begin to look more like the Brain of Cthulhu as it loses health. The Brain of Cthulhu drops the Brain of Confusion in expert mode.
Queen Bee Pre-hardmode Queen Bee can be summoned by using an Abeemination or by breaking Queen Bee's larva. Queen Bee will alternate among 3 attacks: charging at the player, summoning bees while stationary, and shooting stingers at the player while moving. In expert mode, Queen Bee will begin to charge at the player faster as it loses health. Queen Bee will drop the Hive Pack in expert mode.
Skeletron Pre-hardmode Skeletron can be summoned by talking to the Old Man NPC, located at the entrance to the Dungeon, at night and choosing the Curse button. Skeletron can be summoned again if a player kills the Clothier during night while they have the Clothier Voodoo Doll equipped. Skeletron has two attacks: staying above the player while attacking with its hands and performing a spinning head attack. If Skeletron isn't defeated by day, it will act like the Dungeon Guardian, and one hit kill the player. In expert mode, once Skeletron loses a hand or a certain amount of health, it will begin to shoot homing skulls at the player. Skeletron will stop shooting skulls if it is doing its spin attack. Skeletron will drop the Bone Glove in expert mode.
Wall of Flesh Pre-hardmode The Wall of Flesh is summoned by throwing the Guide Voodoo Doll into lava in the Underworld while the Guide is alive. The Wall of Flesh constantly charges at the player, dealing damage if it touches them. Also, The Wall of Flesh shoots lasers from its eyes. The rate it shoots its lasers and moves becomes faster as it loses health. The Wall of Flesh summons "Leeches" to attack the player, and it has minions called "The Hungry" attached to it that deal contact damage to the player. The Wall of Flesh inflicts the Horrified debuff. This makes it impossible for the player to leave the battle, since if the player falls behind the Wall of Flesh or tries to go to far up, there will be a tongue that pulls the player toward the wall. Also, if the player runs too far ahead or tries to teleport away, they'll die instantly. In expert mode, the Wall of Flesh is able to move faster, and The Hungry are able to respawn. The Wall of Flesh drops the Demon Heart in expert mode.
The Destroyer Early-hardmode The Destroyer can be summoned by using a Mechanical Worm. It also has a random chance of spawning during night if a Demon Alter, for Corruption worlds, or a Crimson Alter, for Crimson worlds, has been destroyed, and The Destroyer hasn't been defeated. The Destroyer is like a worm enemy, able to pass through blocks, but all of its sections share one health. It is capable of shooting lasers at the player, and spawning probes that shoot lasers at the player. In expert mode, the head of The Destroyer will do a substantial amount of damage to the player, and it will fire lasers faster. The Destroyer will drop the Mechanical Wagon Piece in expert mode.
Skeletron Prime Early-hardmode Skeletron Prime can be summoned by using a Mechanical Skull. It also has a random chance of spawning during night if a Demon Alter, for Corruption worlds, or a Crimson Alter, for Crimson worlds, has been destroyed, and Skeletron Prime hasn't been defeated. Skeletron Prime has a head and four arms that attack independently, but only the head needs to be defeated. Skeletron Prime's Prime Saw and Prime Vice arms attacks by trying to hit the player. The Prime Laser shoots lasers at the player, while the Prime cannon shoots bombs into the air. Skeletron Prime's head occasionally spins and grows spikes, which causes all of its other attacks to become more aggressive. Skeletron Prime gains additional health and damage in expert mode. Skeletron Prime drops the Mechanical Battery Piece in expert mode.
The Twins Early-hardmode The Twins can be summoned by using a Mechanical Eye. It also has a random chance of spawning during night if a Demon Alter, for Corruption worlds, or a Crimson Alter, for Crimson worlds, has been destroyed, and The Twins haven't been defeated. The Twins have their own unique attacks for each eye, and both of them must be defeated to count as defeating the boss.
Spazmatism attacks by alternating between attempting to charge into the player and shooting cursed flames at the player during its first phase. During Spazmatism's second phase, it alternates between charging at the player, and spewing a stream of cursed flames at the player.
Retinazer attacks by alternating between making short charges at the player and firing lasers at the player during its first phase. During Retinazer's second phase, it will stop charging, and focus on shooting barrages of lasers at a very fast rate at the player.
In expert mode, Spazmatism's cursed flame debuff last longer, and Spazmatism charges faster; while Retinazer shoots lasers faster. The Twins drop a Mechanical Wheel Piece in expert mode.
Plantera Post-Mechanical Bosses Plantera is summoned by destroying Plantera's Bulb while in the Underground Jungle. Plantera has two phases. In the first phase, she slowly moves toward the player with Plantera's Hooks, and she shoots Seeds at the player. As she loses health, she begins to shoot Poison Seeds and Thorn Balls. In the second phase, Plantera changes appearance to a thorny mouth, and she stops shooting Seeds and Thorn Balls. Plantera charges toward the player, spawns Plantera's Tentacles, and spawns spore projectiles during this phase. If a player leaves the Underground Jungle at any point, Plantera's behavior will become a lot more aggressive until the player returns to the biome. In expert mode, the Seeds and Thorn Balls will be able to home into the player, and the Seeds can travel through blocks during the first phase. In the second phase, Plantera's Tentacles will be attached to Plantera's Hooks, and they will have additional health. Plantera drops a Spore Sac in expert mode.
Golem Post-Plantera Golem can be summoned by using a Lihzahrd Power Cell on a Lihzahrd Altar which can be found in the Jungle Temple. Golem's first phase involves jumping around, shooting fireballs and lasers at the player, and attempting to hit the player with its fists. It increases in speed as it loses health. In its second phase, its head detaches and becomes invincible. The body jumps around more often, and the head will continue to shoot fireballs and lasers. The fight ends once the body has been destroyed. In expert mode, Golem's damage and health is increased. Golem drops a Shiny Stone in expert mode.
Duke Fishron Duke Fishron can be summoned at the start of hardmode, but a player typically needs Post-Golem gear. Duke Fishron is summoned by fishing in the Ocean biome while using a Truffle Worm as bait and then reeling in the line as it is bobbing. Duke Fishron mainly alternates among three attacks during its first phase, shooting Detonating Bubbles, summoning Sharknadoes which shoot Sharkrons, and charging at the player. In its second phase, it attacks are altered a bit, and Sharknadoes are replaced with a Cthulhunado which is much larger than Sharknadoes, and a Cthulhunado summons more Sharkrons. It charges less often, and it shoots Detonating Bubbles in a loop shape. In expert mode, Duke Fishron has a third phase where the screen goes dark, and Duke Fishron goes invisible, only able to be seen by his glowing eyes. Duke Fishron will only charge at the player during this phase. Duke Fishron drops a Shrimpy Truffle during expert mode.
Lunatic Cultist Post-Golem The Lunatic Cultist is summoned by killing the Cultists that spawn at the entrance of the Dungeon. The Lunatic Cultist has a variety of attacks that get more aggressive as it loses health. It can shoot homing, exploding fireballs; shoot a ball of ice that fires ice shards; summon an orb that fires bolts of lightning; and fire spreads of Ancient Light projectiles. The Lunatic Cultist can preform a summoning ritual to create decoys of itself which fire shadowflame attacks. The decoys will not spawn if the player attacks the Lunatic Cultist. If a duplicate is attacked or the player takes to long to attack, a Phantasm Dragon will be summoned. If The Lunatic Cultist does the ritual again and the player fails to stop it while the Phantasm Dragon is still alive, an Ancient Vision will spawn and attack the player. In expert mode, the Lunatic Cultist will also spawn Ancient Doom projectiles. The Lunatic Cultist doesn't have any expert mode exclusive drops.
Moon Lord Post-Golem The Moon Lord can be summoned by defeating the four Celestial Towers or with a Celestial Sigil. The Moon Lord has a variety of attacks, such as summoning Phantasmal Eyes, Phantasmal Spheres, and Phantasmal Deathray. The Moon Lord has too many attacks to cover in this short space, so visit the Official Terraria website for more information on the Moon Lord's attacks. In expert mode, the Moon Lord drops the Gravity Globe and Suspicious Looking Tentacle.