Guide:King Slime strategies

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

Terrain Preparation
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.
 * Since 1.3 update King Slime can teleport to you at will so hiding in houses will be impossible. This also makes platforms useless. Nearly all strategies below are rendered useless as of 1.3. It might be best just to set up a decently large and flat area for battle.
 * 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.
 * You can make a simple room 4x4 with wood platforms as a celing.The slimes won't go in and you can shoot at him with any Ranged weapon as them can shoot trough the platform.

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.

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.

Potions

 * Ironskin Potion can help reducing the damage received even further.
 * Regeneration Potion
 * Any food that grants Well Fed buff.

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.