Guide:King Slime strategies

The King Slime is an boss in pre-hardmode. Unlike most bosses, this boss can't pass through blocks.

Terrain Preparation
'''Note: Since the 1.3 update King Slime can teleport to your location if separated from you by a wall. Here is a way to defeat him easily:'''

1.3 Strategy: Due to the fact this boss can't pass through blocks, there are some arena options: It is also recommended to close any caves near the summoning spot, since the boss might spawn inside and get stuck. Another strategy is to make something that will help you later, a long minecart rail path. If you go high enough, he will not be able to teleport or jump on you. His attack pattern relies for you to be on the ground. To the AI it seems that you are on the ground, so he just stands under you and occasionally jumps, making him an easy target. For an even easier battle, you can make the space gun and meteorite armor, so you can deal lots of damage while he cannot damage you. It also saves you money, as when the Meteorite Armor is used, the Space Gun does not cost mana.
 * Build a one-block thick wall 18-20 blocks high
 * Create a walkway between 35-50 blocks long with platforms for maneuverability.
 * Make sure a Campfire is nearby.
 * Use any Ranged or Magic weapon in your inventory (Yo-yos will work as well).
 * Best example of strategy: https://www.youtube.com/watch?v=g7_LNVl2teI
 * The player can add traps to this pit, which would make the fight even easier or:
 * A flat area with small room below it with Platforms as roof for the player. King Slime can't enter small rooms and thus can't touch the players in this room.
 * An arena made up of platforms and Ropes for both horizontal and vertical movement would help. (Effective for 1.3)
 * Or, just make a small 2*4 pit cover the top with wooden platforms and shoot upwards.

Because the King Slime can be harmed by lava, a good strategy is to use a lava bucket to make a long strip of land with a small amount of lava (shallow enough so items don't burn) and then make some widely placed platforms. You can jump, use rocket boots, or use a grapling hook to get from platform to platform. (Simply jumping is not recommended, because then you may fall, or the gaps will not be far enough for the King Slime to fall through and get hurt from the lava.) This strategy works with the slimes teleport's, and the minions will all die from falling in the lava also. You don't have to focus on fighting, because the lava will dish out major damage. You can also skip the platforms, and use a obsidian skin potion, and just evade by running under the slime, or using boots to fly over.

Armor
Lead armor/Iron armor or higher is recommended to reduce the damage of Blue Slimes summoned by it to an insignificant amount. However, this boss isn't hard without armor, if using a good arena.

Weapons
Most weapons can be used to fight this boss without difficulties, except Shortswords. Piercing weapons may be preferred, to hit both the blue slimes and the king slime himself.

However, melee users may have trouble fighting the boss due to the knockback immunity, unlike ranged users which can stay in an unreachable place without getting hit. Recommendations:
 * Gold/Platinum Broadsword or better.
 * Lead/Iron Bow or better with piercing arrows such as Unholy Arrow or Jester's Arrow.
 * Any kind of Guns, Spears, or Flails.
 * Vilethorn, this weapon hits through blocks and pierces infinitely until the duration ends and therefore very good for fighting King Slime.
 * Crimson Rod, if using a small enough pit, the king slime will stay in one spot and this weapon can do a lot of damage.
 * Any weapon that can go in one direction, such as a Gem Staff.
 * I used the crimson yo-yo, and it seemed to work well with the string and counterweight accessories, because of the good range and the DPS it can deal.

Accessories
Like weapons, most accessories can help:
 * Shackle can help reduce the damage of the boss and his minions.
 * Hermes Boots, Flurry Boots, or Panic Necklace can help avoiding damage or running away in bad situations.
 * Cloud in a Bottle, Blizzard in a Bottle, Sandstorm in a bottle, and/or Rocket Boots can also help with vertical movements.
 * If you have defeated the Eye of Cthulhu in Expert Mode, the Shield of Cthulhu can be useful both in damaging the King Slime and minions, and making a quick escape.
 * A Band of Regeneration can be useful to increase rate of regeneration while avoiding being damaged by King Slime

Potions

 * Ironskin Potion can help reducing the damage received even further.
 * Regeneration Potion
 * Any food that grants Well Fed buff.
 * Swiftness potion, so you can evade the teleports and run under the jumps if needed.

The Battle
Fighting King Slime is relatively simple, it will attempt to hop to the player and summon blue slimes. Keep killing the tiny slimes as soon as possible and damage the king slime after that.

If you are cornered, try to pass through the boss to the other side of the arena by running while it is jumping at you.

Expert Mode
In Expert Mode not much changes about the King Slime itself besides its higher damage and HP. However, when damaged it will spawn Spiked Slimes in addition to regular Blue Slimes. Those can throw volleys of spikes at a short distance, so having a weapon which can pierce multiple targets and attack from a range (such as Throwing Knives) will be necessary to avoid being bombarded to death.