Now that you're fighting, there are some things to know about the king slime. Now that you are sure you're ready, it's time to fight. It is best to prepare an arena to fight it near the ocean. If the chests have speed increasing boots, then continue after equipping the boots. Next, try finding a ruined home underground in either the ice biome or purity biome. Remember to stock up on mana potions! It's also good to get Archer potions, if you are a ranger. If you cannot get the waterbolt, a reasonable alternative would be a diamond staff, and a diamond cloak. If you're playing mage, try finding the water bolt in the dungeon, chances are there may be one above the limit before the dungeon guardian spawns. It's also recommended to get some bombs from a demolitionist (you should have one at this time,) and get the world evil's gun if you're playing ranger. That means you should have pretty good weapons, 200 health, gold armor, a heart bracelet, and a shackle. Try to get at least the minimal requirements for the Eye of Cthulhu to spawn naturally before going up against this guy. This boss has a large amount of health and attack power, so get ready for the worst. Mage and Ranger are recommended, as they can hit from a distance without doing minimal damage. It's not easy fighting him with summoner, either, because at that point and time in the game, if you have the slime staff, you'll only be able to summon one. This dude is challenging to tank, as he can deal a huge number of damage.
It's best not to go melee unless it's YoYos.
But the question is how do you beat this thing? This is likely the first boss you'll fight in Terraria, because exploring is a huge thing in the game, and you might go to the ocean for multiple things. Obviously, this slime has been able to take down people before you, so you seem just like a snack to it but not quite. Suddenly, a massive slime wearing a crown with a ninja trapped inside attacks. Imagine one day you're fishing close to the ocean, then, your game tells you: "The King Slime has awoken!" Of course, curiousity strikes you and you go to see who this "King Slime" is.
There are many things you can do in Terraria, from mining, to fighting, to even fishing.