King Slime is one of the easiest bosses, and one of the first a player will beat in a playthrough. To beat King Slime, I suggest for:

rangers: gold/platinum bow
summoner: the snapthorn, and flinx staff or vampire frog staff
mage: ruby staff or better
melee: amazon or enchanted sword

I suggest for:

ranger: minishark or gold/platinum bow
summoner: the snapthorn, and flinx staff
mage: vilethorn or diamond staff
melee: blade of grass or amazon, or thorn chrakram.
For expert or master mode, it is a must to have hermes boots to dodge the dash attacks in its second form, the last 50% of its health.

This boss can be easily beaten if you have a piercing weapon. I suggest for:

ranger: gold/platinum bow or demonite bow, with jester arrows
summoner: snapthorn, with the vampire frog staff
mage: the vilethorn
melee: blade of grass or starfury, or the ball o'hurt
when the boss is in a straight line, a piercing weapon does mass amounts of damage.

For the first stage, before all the minions have been defeated, it is best to use a piercing weapon, or a weapon that hits multiple targets. I suggest for:

ranger: crimtane/gold/platinum bow with jester arrows or minishark
summoner: snapthorn, with the vampire frog staff
mage: space gun or gem staves
melee: rotted fork, or blade of grass

I suggest for:

rangers: molten fury or boomstick
summoner: snapthorn and imp staff
mage: space gun or demon scythe
melee: blade of grass, or starfury.

I suggest for:

rangers: bee's knees and beenades or minishark
summoners: snapthorn with the hornet staff or imp staff
mage: space gun or bee gun
melee: blade of grass or crimson/corruption yoyo

This boss is the only mandatory boss in pre-hardmode (in terms of progression) and if beated, changes the world into hardmode. I suggest for:

rangers: hellwing bow, or the bee's knees along with beenades
summoner: spinal tap with imp staff and ballistia
mage: flamelash, aqua scepter, or demon scythe, or flower of fire
melee: nights edge, or sunfury

Pre-mech Duke Fishron is a hard bossfight, but is doable with the right set up. Achieving this can give you some of the best gear, and carry you almost the whole way through hardmode. The fishron wings are the best you can get early hardmode, and the tsunami bow can carry you to the lunar events, which is the same with the razorblade typhoon, and the tempest staff is a good option.

Weapons for this fight:

ranger: Daedalus stormbow with ichor arrows and/or holy arrows.
summoner: sanguine staff and cool/firecracker whip
mage: spirit flame and golden shower
melee: bananarang and/or shadowflame knives

Accessories for this fight:

Amphibian boots
frozen wings (or equivalent)
shield of Cthulhu
worm scarf or brain of Cthulhu
charm of myths.

For class specific accessories:

ranger: Stalker's quiver, ranger emblem
summoner: Pygmy necklace, summoner emblem
mage: mana cloak, sorcerer emblem
melee: power glove, warrior emblem

Armour:
ranger: titanium/adamantite armour
summoner: spider armour
mage: titanium/adamantite armour
melee: titanium/adamantite armour

Arena setup:
In the ocean, make a planter box platform from the beach to the end of the world. Then fly up and use the ice rod, to make a floating block, and then place planter boxes on either side of it. Make sure that you can reach the 2nd platform easily, not too high that you can't reach by simply flying. Add campfires, and heart lanterns.

Tips:
Tip 1: In expert or master mode, use the shield of Cthulhu to dash into fishron, if you do not have time to dodge, as this will stop you taking damage if done right.
Tip 2: Also in expert or master mode, in the third stage, where fishron isinvisible, then dashes towards you, wait until it is about to dash, when you can first see its eyes, jump or go downwards, if in midair, to dodge the attack.

To spawn plantera, you need to break a plantera bulb. These can be quite rare and annoying to find. I recommend that you make a you make some lines of mud with jungle grass, near an arena. This will spawn plantera bulbs given enough time, along with some life fruit. For the first phase of the fight, just simply circle the boss, and you should avoid taking any damage. For the second phase, the boss becomes faster and harder to dodge. Using a rod of discord can be helpful to dodge the attacks.

The goblin army is a pre-hardmode event that spawns after:
at least 1 player has 200 health
at least 1 Shadow Orb or Crimson Heart has been destroyed
This makes it 1/3 chance happening every day, after one has been defeated it lowers to 1/30 chance.

This event happens when you place 100 torches in a single area. It shoots the torches at you to deal damage. To beat it, you need to dodge the torches, and if you win, you get the Torch God's Favour, a consumable item that auto changes torches you place into the torch from the biome you are in, e.g. jungle torch in jungle biome.