Part 1: First Bosses
Your first boss will be King Slime. He’ll either spawn naturally after a slime rain, or you can manually summon him using a Slimy Crown which uses 50 gel and a gold (or platinum) crown, which requires 10 platinum/gold bars and one ruby.
For the fight, I would make a platform (out of wooden platforms) about 500 blocks long. You probably won’t need 500 but some of the upcoming fights may – it doesn’t hurt to be safe. Also make a safety platform below the original platform, so that you don’t get bombarded by mini spiked slimes. After you kill him, you should immediately equip his expert drop, the ‘Royal Gel’, it makes you immune to slimes, which is always helpful.
Now that you’ve got the Royal Gel equipped, wait until night-time and then fight The Eye of Cthulhu. The Eye of Cthulhu can be defeated with a simple bow and arrow. I suggest using Flaming Arrows to do some extra damage. Make sure to put Campfires on your platform for healing. At this point, having Hermes Boots and a Cloud in a Bottle for extra mobility is very helpful. Also for the Brain of Cthulhu, bring some bombs to the Crimson. Destroy two orbs, get the gun that drops, wait for the arms dealer, and buy a mini shark for the arena. Then build a box with platforms and do circles around the brain until you kill all the eyes, then stand in the middle of your arena and keep shooting the real brain.
Part 2: Hell, and the Jungle
Assuming that you have finished your Hellevator, a hole that goes from the surface straight down to the Hell biome, you should now have quick access to Hell. Before you jump down to go explore, I’d first recommend making more houses in each biome for Pylons (Pylons are teleportation tools) and put the Cavern Pylon right down the bottom of your Hellevator. For exploring Hell, the equipment that you should have (my recommendation) is the Lava Charm (Terraspark Boots are obviously better), Frost Spark Boots, and the Royal Gel. The Lava Charm will help you get a bit of Hellstone, but an Obsidian Skin Potion is best. You can find them inside of pots in Hell. This will get you lots of Hellstone. If you didn’t get enough Obsidian while making your Hellevator, you can go to the jungle to get more Obsidian. (Go there anyway and finish looting up.)
IMPORTANT! Explore the jungle a lot while looting, or you will be in pain after the Wall of Flesh. Get Hellstone armour for Skeletron and fight him, he’s the only other boss that I struggle with, so I don’t have any recommendations for him. After beating Skeletron, you can go down into the dungeon and try find the Muramasa. When you find it, you can leave the dungeon and make the Night’s Edge. After you make a Night’s Edge, you should easily be able to beat Queen Bee. For her arena, make a giant box near a hive, put platforms in your hook’s reach and you should be good to go. Also, if you find a Bezoar it will become very helpful — it is also worth grinding for if you don’t have it.
Part 3: Wall of Flesh/Early Hard Mode
For the final boss of pre-Hardmode, you will need to build a giant arena in Hell, spanning from one end of the world almost to the other. Make four of these so you can ‘cheese’ the Wall of Flesh. By doing so you won’t get hit by the lasers, making this fight un-losable. Also, just in case, I would take some potions just for the guaranteed win.
After beating the Wall of Flesh, you are now in Hard-mode. Congratulations! This is where the pain starts… First, grab the Pwnhammer and go to your Crimson/Corruption and smash all the altars (make sure you have at least one accessible altar near your new spawn.) Once you’ve broken all (but two) of the altars, some new ones will spawn. There are six ores. I like to split them into Hardmode Ores One, Two, and Three. There are two ores in each section, and your world will be blessed with one ore from each section. For Hardmode Ores one, you can either get Paladium or Cobalt. For Hardmode Ores two you can get either Orihalcum or Mithril, and for Hardmode Ores three you can get either Adamantite or Titanium. Slowly (or quickly) make your way to Hardmode Ores three so that your gear is either Adamantite or Titanium.
Another fun perk that you unlock from Hardmode is wings. You can now obtain wings by killing Wyverns. They are found in the space layer, and by combining the Souls of Flight with some other ingredients. For wings, I recommend getting the Pixie Wings, because not only do they have 67 blocks of height, but they are also a good source of light. These wings require 99 Pixie Dust and some Souls of Flight. These will be your wings until post-Golem (a long way away).
Also, another thing to keep in mind is that the Hardmode Ores one, two and three are for all classes, but after those bars there’s Chlorophyte bars for Melee classes, Shroomite for ranger classes and spectre bars for the Magic Class (R.I.P Summoner.) AND, after all these there are the Luminite-infused armour (post-Moon Lord) even further away.
Part 4: Hardmode Bosses
For the ‘first’ boss, Queen Slime, I usually don’t fight her considering that she doesn’t drop anything significant for any stage of the game, so the first boss that I would start with is The Destroyer.
For The Destroyer, you should use the Fetid Baghnakhs, which you can obtain by killing a Crimson Mimic. To spawn a Crimson Mimic, you’ll need to make a Key of Night from 10 Souls of Night. Once made, place the key in any type of empty chest and it will spawn. For The Destroyer fight you can stand next to The Destroyer and just melt him. When the fight starts, for the first like 10 seconds he gets 99,999 armour, so you’ll only deal one damage, but when that defence is gone the armour will drop a lot, so when you summon The Destroyer, run to his clumped-up body and you can deal a ridiculous amount of damage really quickly, leaving The Destroyer at about half health, making the fight simple from there.
The Twins, they’re the hardest Mech Boss in my opinion because there are two bosses chasing you, but the best way to kill them is to make a long platform and run from side to side using a weapon with projectiles.
And finally, for the last Mech Boss, Skeletron Prime, for him I find that I never die to him, but I run out of time. So, my advice for him is just to start the fight as soon as night comes. Apart from that, this fight is also easy.
And after all that chaos Planterra’s bulb will spawn. Before you fight Planterra you should go to the dungeon and remove all the spikes before it gets tough, then find Planterra’s bulb (which is a bright pink two by two square on the map, which you’ll find in the jungle). For the fight, all you need to do is do circles around Planterra and you’ll win. But look out for her spiky balls that will follow the player. Keep in mind that the balls have gravity, meaning they will follow you, not chase you. After Planterra is defeated, make sure to find lots of Life Fruits so your health can reach 500.
Part 5: Golem, Empress of Light and Moonlord
After beating Planterra, you can now enter the Jungle Temple (if you haven’t hoiked into it already!) You can find Lihzahrd power cells used to summon Golem at the Lihzahrd Altar, found at the bottom of the temple. For the fight, I have no recommendations, except for when his head detaches, do not focus on the head, although it’s easy to kill, and it’s most likely the main source of damage, if you kill the head, it will enrage the body and it will kill you. So, aim for the body and that will just finish the fight.
As for The Empress of Light just do circles around her, but mostly focus on dodging her attacks. For Daytime Empress, don’t try. You will just die, and you don’t even need the Terraprisma, so there’s no point.
And finally, for the Lunatic Cultist, I don’t have many strategies for him, but I wouldn’t use a homing weapon, because it may focus on a fake clone, causing your dodging to become twice as hard. After beating the Lunatic Cultist, the pillars will spawn. For the order of pillars, start with the Solar Pillar for the easy melee weapon (get the solar flare for pillars and the daybreak for the Moon Lord). After beating the Solar Pillar, all the other pillars are easy. Just box yourself in and use the Solar Flare through the blocks until the shield breaks.
Finally, to finish the game you will need to kill the Moon Lord, the final boss in Terraria. To do so, you’ll need to make a platform out of asphalt about a thousand blocks long. For the fight you need to focus on blocking his attacks. Jump when he sends his lasers and when the eye on his head opens, do a circle around him to dodge the laser and start running the other way. Continue this until his heart gets exposed, then swap weapons to the Solar Eruption, deal insane chain damage to him and then GGEZ.