The Twins are the hardest, according to most people. There's not really much of a good strategy to kill them very fast, except to kill Spazmatism first and run/fly fast. You could also use the Golden Shower on them.
The Twins are a Hardmode mechanical boss which is essentially a more difficult version of the Eye of Cthulhu.
6/13 The Twins
This boss is arguably, probably, the hardest of the three mechanical bosses in Terraria. The Twins are, in a nutshell, two bosses in one with the two separate eyes - named Retinazer and Spazmatism. They each own have an attack pattern and life count. Both must be killed in order to defeat The Twins!
The Twins can be a difficult hard mode boss to beat in Terraria. Start with step 1 to learn how to beat them with not too many 'high-tier' items easily.
The Destroyer itself will only deal relatively low damage, less than your healing potential, making the fight very easy. Additionally, you can put a few Heart Lanterns and use Regeneration Potions to increase your regeneration rate.
In Normal Mode, Destroyer is the easiest and Skeletron the hardest.
It's recommended for experienced players. The Destroyer is a very fast stable long distance driver. This disc is much like a faster Wraith, but with a little more high and low speed stability. Great disc for sidearm throwers and those with lots of power.
Summoner, in my opinion, is the worst class. He has the lowest defense and has good damage against enemies, but his defense is so low he dies extremely easily. He can't do much against bosses of any kind if he doesn't have endgame gear.
Crystal Bullets and Ichor Bullets are a must-have if using guns, and Holy Arrows and Ichor Arrows for bows. Ice Rod is useful against Retinazer because the spawned ice blocks can block its lasers.
Journey Mode: This is an easier mode, that gives the player extra items on the start, allows duplicating items, and allows making the game easier.
The Destroyer is a colossal mechanized Eater of Worlds with 80,000 / 120,000 / 153,000 health. It is considered the easiest Hardmode boss by many players, despite having the second-highest health.
Although you could fight Bomb Man first, I recommend taking care of Guts Man right off the bat because he's rather easy if you dodge his attacks effectively and his weapon is useful in some of the stages.
With a more than respectable base damage of 119, the standard Twinblade is a great early-game weapon, and can be found fairly easily at the start of the player's journey. Its scaling options are also solid, and its Spinning Slash weapon skill is a nice addition, but it's just edged out by the rest of its class.
What are the best perks for The Twins? A deep bond with The Entity unlocks potential in one's Aura-reading ability. After hooking a Survivor, all other Survivors' Auras are revealed to you for 4 seconds when they are farther than 40 metres from the Hook.
While the Nymph is one of the rarest enemies in Terraria, in reality, she spawns more frequently than is initially apparent. She is usually missed due to her passive and stationary behavior: she often remains in dark areas, without moving to attack players or making any sounds, unlike what most enemies would do.
The Code 1 is the strongest pre-Skeletron yoyo.
If you have the full clorophyte armor set, you will get a bonus crystal leaf, which does 40-50 damage every hit, which will be useful because of the destroyers long body. You will also need wings, like the bee, demon, or angle wings which go 72 blocks and above.
The Destroyer is a Hardmode mechanical boss which is essentially a more difficult version of the Eater of Worlds. Like the Eater of Worlds, it is a very large worm.