An Arena is generally a large enclosed area where the player can face a challenge in an environment they can control. It is generally designed to keep most enemies out while focusing on a Boss or other significant challenge. each arena can be different for each boss, some consisting of a very long line with wall at the end or more complex ones with ledges, traps and statues. players should keep the arena about a screens height off the ground so there won't be any unwanted wyvern fights during a bigger fight
A highly recommended project for players who are about to summon their first boss. An arena for the Eye of Cthulhu should be large enough to dodge and maneuver around in, but small enough to seal off any enemies from wandering in and making the fight more difficult. A recommended size is just about the width of the screen.
Adding in a roof is also recommended to protect from airborne enemies and to keep retrievable ammunition from flying too far away. Making several wooden platforms can make dodging simpler. As the fight will be at night, add torches to help visibility, including several in the ground underneath the arena.
Some players may choose to abolish the roof, and keep the Arena open to the sky. The reason being that there is a chance (however small) that Fallen Stars will hit the eye and deal 999 damage. This can make the fight far easier.
Location is important for an arena of this sort. Specifically, the player must be in The Corruption to summon the Eater of Worlds, and moving too far away from The Corruption during the battle will cause it to flee. If there is corruption close to your base, then using it is recommended. Otherwise, artificial corruption using Corrupt Seeds and/or Ebonstone is necessary. An area of just corrupted dirt will work sufficiently to summon the Eater of Worlds.
The Eater of Worlds is, of course, a burrower, and will usually be offscreen and underground for much of the fight. A properly constructed arena can remove much of that advantage. Make the arena fairly tall, thin, and easily climbable with many wooden platforms. That way, you can not only see it coming, but this will force it to climb in a relatively straight line, leaving large portions of its body exposed, and vulnerable to piercing ammunition.
The above setup assumes you are using Worm Food to summon the Eater on the surface. If you are breaking Shadow Orbs, hollowing out an entire arena every third orb or escaping to a surface arena may not be practical; you should still modify the terrain around you to have a clear path of escape from the chamber where the Orb is located and ideally, easy access to a cleared space with good horizontal clearance and any hazardous terrain smoothed out or boarded over. Alternatively, if you have found a Magic Mirror, you can build a small bedroom at the top of your arena. Set it as your spawn point before descending into the chasms, then you can use the Mirror to return to the arena to fight after breaking every third Orb.
The entrance area to most dungeons can often be modified into a more desirable arena if needed. Most important is that you have room to dodge the hands, as well as enough vertical space to avoid getting trapped when Skeletron's head begins to spin. Walling the area off might also help exclude any minor enemies from joining the fight.
Ensuring the floor is nice and flat will also allow plenty of time to take advantage of the Skeletron's spinning attack, as its defense is set to 0 while using that attack. It may also be a good idea to bring along any Spiky Balls to set on the floor, if you have any from fighting the Goblin Army. When Skeletron begins his rolling attack, he'll roll right over them and take the full brunt of their damage.
Location is important for this arena as well. The goblins will naturally target your initial spawn, that is, the center surface of the map. So no matter what you actually have there, that's the general area where you will encounter goblins when they arrive. You may as well prepare a proper greeting for them.
Make sure there are no NPCs in this area, as they will be attacked and possibly killed during the fight. Goblin Sorcerers' attacks go through walls and they can teleport indiscriminately, so the only defense is to make sure that your NPCs are not nearby.
Flatten out an area and make sure that there are no pits or jumps. Walls are not necessary, as during an invasion the newly spawned monsters are the target rather than a distraction. Make certain that any platforms or areas can be easily and quickly reached from anywhere else on the screen, since most players will be looking to quickly close in on casters with their melee weapons.
Though this boss is particularly easy, and an arena probably isn't necessary, it's possible to exploit this creature's unwieldy size and movement pattern to create an area where it is impossible for the player to take damage during the fight. Create several "foxholes" poking up to the surface, large enough for the player to jump (and shoot) through but too small for the boss to travel in. Then connect them all with a tunnel.
Or you can build a 8 tall satellite with an open bottom and 4 layers of platforms and rapidly fire at him while the extra layers catch the extra blue slimes.
If you can manage to trap the King Slime it is possible to defeat him extremely early in the game with a copper pickaxe, if you have enough time to go to the outer thirds of the map, though it takes an extraordinary amount of time.
The Wall of Flesh, more than probably any other boss, benefits from having an arena prepared before engaging. Even experienced players wishing for a quick fight should ensure that the path behind them is clear before summoning him. A full arena can speed up farming this boss, which can provide the player with several weapons and accessories that are valuable even late in the game.
The simplest arena is a long bridge of about 750 blocks with some space above it for the player to maneuver. Placing the player out of danger from falling into lava and ensuring that they can keep a steady pace of progression allows the fight to go quickly and smoothly. Players may also wish to place lights along the bridge, or place markers along the bridge at points where they've killed the WoF in the past.
Note that Underworld map generation tends to place Ruined Buildings towards the middle third of the map, leaving the eastern and western wings empty aside from Ash, Lava, Hellstone, and plenty of enemies. Lava topography also tends to level out at either end of the screen within a ten-block height variance.
The Wall of Flesh begins its attack from whichever side of the Underworld's map edge you're closest to. This allows you to control its direction of approach. The minimap on the right side of the screen can interfere with visibility on the Wall of Flesh at times. Fighting west-to-east may be easiest, though once you are familiar with the mechanics the direction won't make a large difference. Fight in the direction you're most comfortable running.
Given all the above, an optimal Arena can be made by creating a long Ash bridge starting from one side of the screen and running long and flat for a distance of 500 to 1000 blocks, depending on your damage output. Going from the edge of the map to the first Ruined Building you see will give you a sufficient platform, but on a Large map that may be a punitive amount of construction work for the player. Wooden Platforms are not useful in this fight as the Wall of Flesh will constantly orient to the Player's position
Clear any Ash up to three blocks above your bridge (standing overhead swing height) so you have room to jump occasional lasers. Level any Ash hills in your way and drain lava appropriately. It is very difficult to run out of Ash - Players should feel comfortable using explosives to create retention pools and drainage channels.
Killing the Wall of Flesh will generate a bricked-in treasure box above your bridge. The bricks can serve as kill-markers, but they may also interfere with jumping if your next fight runs close to that spot. The bricks can be mined for your collection to keep a clear Arena.
Your Eater of Worlds arena should also serve as an arena for fighting The Destroyer (However, moving the walls and ceiling in may protect you from of the Destroyer's lasers). The same types of visibility and movement are needed for both fights. The Destroyer does not need corruption present to be summoned.
Before fighting the twins it might be a good idea to build a lot of platforms and make it easy to move around. When you're fighting the Twins focus on killing Spazmatism first. His attacks deal a high amount of damage, and they inflict the Cursed Inferno debuff. Also Focus on killing one eye and then the other. Having both eyes on their second phase can make the fight a lot more difficult.
Building an arena for fighting Skeletron Prime can be a little tricky. Simply using Gravity Potions and fighting in the sky may be more effective. A long bridge may also work. (Use the arena for the twins if you have one) The explosive only detonates on solid blocks so use some wood platforms as well
An effective arena for the Golem would be to square out and level the room that the Lihzahrd Altar is in and use wings to fly around and dodge his attacks (mainly magic and ranged users). While exploring the Lihzahrd Dungeon, you can gather all the traps. Construct vertical pillars out of traps large enough for Golem but high enough for him to not get out on both sides of the altar. Finally line the ceiling with spike ball traps. After summoning Golem leave the enclosure and repeatedly activate traps to help fighting Golem (Be careful for the spiky ball traps, as they still hurt you)Also, using weapons such as the Death Sickle or the Mushroom Spear are useful, as if you make a walled-in arena, you can hit him through the walls(Frost Hydra staff could also be used).
Build a long hallway, preferably about six blocks high and 100-200 blocks wide, with teleporters at each end of the hallway. When you spawn Plantera, have it go through the hallway, and run to the edge of the hallway to one of the teleporters. When Plantera charges after you teleport to the other end of the hallway and shoot down the hallway, with some sort of non-arced, high velocity weapon, such as the Megashark with either cursed, or crystal bullets. Adamantite, or Hallowed armor is suggested for this fight, with no preferred accessories. Repeat this process until Plantera has been defeated.
Farming Meteor Heads can provide large amounts of coins. An "artificial" meteor biome can be created by placing exactly 50 meteorite in close proximity to each other. This can be done inside under controlled circumstances to farm these creatures. To deactivate the meteorite, removing only one piece is required. This method is not incredibly efficient as the Meteor Heads move very slowly.
Fire Imps, as anyone who has fought one knows, teleport quickly and shoot fireballs that pass through walls. When they are hit once, they stop firing and pause at their present location for a number of seconds, leaving them vulnerable. Creating a long horizontal enclosed tunnel about 4 tiles high, with a good deal of empty space above and below, will assure that the Fire Imp will usually spawn within reach of the Vilethorn, which can attack through walls. The Vilethorn will take some time to kill the Imps due to their high defense; you can opt to use the Vilethorn just to stop the Imps from attacking, then wait for them to teleport within your tunnel (which will usually happen on the second or third teleport), so you can hit them with the Thorn Chakram, which will kill them very quickly.
Demons can be farmed in a similar way to Fire Imps, though it is important to remember that the Demon's projectiles can damage the player through one-block-thick walls, if you're in contact with that wall. In case Demons spawn below the platform you're standing on, you'll want to make that platform 2 tiles thick. Also, if Demons spawn below your tunnel, you will want to have solid ground or another platform built far below your tunnel to catch dropped loot (anything but lava). Demons will always fly as close to you as they can, putting them well within range of the Vilethorn, which will dispense with them rather quickly.
Ocean enemies can be farmed by building a long bridge over the ocean. this is a good way to farm Pirate Maps, glowsticks, shark fins, Black ink, and, of course, coins. Build two layers of platforms - One 2 blocks under water, then a layer on top of the water. Above this stand in a small pool of water so that all of the mobs swim up towards you. This will cause water mobs to jump out of the water onto the platform. The Spider Staff is good for automatically killing the mobs as they obediently stay out of the water and is very easy to obtain. Another way to kill them is with Spear Traps or Spikey Ball Traps from the Lizarhd Temple. It is recommended to put mushroom grass mud in a large square above the ocean. This will cause Fungo Fish to spawn. Some Lizahrd Bricks in a large square, too. This is for Aparimas. Some Crimstone would be nice for blood Jellies, too!
- Leave campfires, heart lanterns and heart statues linked to adjacent pressure plates in order to have improved self healing during the fights.You can also make a honey lake with platforms on top so you can just hop down for a quick regen.You can put your pressure plates (Above & Below) in the lake.
- Make a house close to the arena and move the nurse there and then set it as your spawn point, that way you can use the Magic Mirror to escape from a pinch and get some healing then return to battle.
- A good way to regain full health is to build a house for your nurse and put it away from your arena, and connect your arena with the house with a teleporter and when you are on low health during the battle, you can teleport to your nurse's house to buy a heal and teleport back to resume the battle. (Be careful as building the house too far away may cause the boss to de-spawn)
- You could even add your unwanted dart traps on the side linked to pressure plates for extra help in battle. A good idea is to link them to the same pressure plates that your heart statues are linked to. Players in later stages of the game can loot the Temple for super dart traps, fire or spear traps to use.