The Twins is one of three Mechanical Bosses in Terraria's Hard Mode. This Guide to defeating them will focus on what you need to prepare, how to get the components for the mechanical eye to summon them at will, and what you'll get when you defeat the pair. Like all Hard Mode Mechanical Bosses, they may visit you at night randomly and you may get the unique summoning item from any kill so long as it hasn't been defeated yet.
Random Encounter Before Summoning
If you've not yet defeated this boss, there is a slim chance each night that they will spawn and attack you. The message for The Twins is "This is going to be a terrible night..." .
Crafting Mechanical Eye
As with other Hard Mode bosses, once you have the required item crafted at your anvil, you can summon The Twins at night time.
Recommended Gear Level
With the below boss fight arena setup, I was able to do this in Mythril Armor (with Melee helm for more defense), so if you've got Titanium you should be even better off. I did this with all accessories reforged to Warding (+4 Defense). With Titanium gear, you can get by with some +%Crit or +%Damage to make the fight a bit faster. I had a total 67 defense. On my last run, I had the following accessories:
Preparing an Arena
I use a long platform or stretch of ground for this fight, so that I have plenty of room to move. You want this platform to stretch 4-5 screens, because you will need to turn around to go back the other direction at the end, and that can be dangerous. I took the extra wood and made a second level about 10 blocks below, so that if necessary I could drop down to avoid damage, then go back to the top ramp. Looking at how they behave, you'll see why. Sprinkle Campfires and Heart Lanterns along the platform so that you have the aura no matter where you are - this will buy you a couple hundred extra life over the course of the fight. You can place a heart lantern on a platform by placing a single normal block atop the platform, then breaking the platform underneath it. This gives you a spot to hang the lantern. This fight is worth using all your best potions on. Get well fed as well, by simply eating some food from the traveling merchant, pumpkin pie by buying seeds from the dryad, or cooking fish if you've got 1.3.
I highly recommend avoiding Melee for this fight. The Twins have a tendency to kite you, and you'll spend more time chasing them than doing damage. At times, they will come in close to charge you, but it's not an opportunity to do big damage - rather, you'll likely take it. Now, if you've got a good ranged melee weapon, go for it. I used a Titanium Repeater and Frostburn Arrows for one fight (in Mythril Armor), Shadowflame Hex Doll and Megashark for another, much easier fight (in Hallowed). In the first case, I had not defeated any other mechanical boss and just wanted to see if Titanium was really necessary (it's not, with plenty of combat potions).
The Fight with The Twins
There are two Twins, of course, and both are different in how they attack. Both have a tendency to charge at you at times, but spend the majority of the time spreading out and avoiding getting into Melee range - yep, they kite, a lot. It's best to go ranged for this fight for that reason.
If you do not keep mobile during the fight, they sometimes get on opposite sides of you and fire off their attacks. The laser doesn't hurt much, but definitely watch those green fireballs. For this fight, staying on the move is important so you must work on your ability to aim while at the same time hitting a moving target. Thankfully, they'll sometimes hover.
The Twins are named Retinazer and Spazmatism. Kill Retinazer first - his berserk mode is to fire more lasers, while Spazmatism will go berserk and utilize a flamethrower - heavily - to the point that it's more dangerous than its brother. Thankfully, the flamethrower attack slows him down. Given the tendency to charge, you'll see why I recommend having some room to run. You'll also find a lot of their attacks miss as they do not fire their shots while accounting for your movement. You'll need to aim carefully as you run, but with you having this capability and them lacking it, you have an advantage.
This was my key mistake. I was originally killing Spaz first, he'd go nuts on me while the other continued to charge, and it sometimes led to them getting double hits on me. That flamethrower is nasty, and Ret has less life and defense than Spaz. More importantly, you should not find that you've got both of them down to half health so they both berserk.
Now the reason for the long platform. When they get angry around half health, they'll get a boost to defense and start utilizing special attacks. Running while firing backward, you can do a lot of damage, then jump when they get close, then utilize your wings to leap up and around them to start moving in the other direction - back and forth on your long path. Spectre Boots and the Unicorn mount both work very well for this, but you can compensate for either with a Swiftness Potion. You may even fire diagonally from midair and do respectable damage as you switch directions. Wings are incredibly helpful, but in a recent game I proved they aren't necessary by just using Spectre boots to rocket over them.
Drink your potions when you're missing more than 150 life, so that you can get the timer started on potion sickness. That's about a minute you cannot use a potion, and it's better to do this before you're under 200 life for that reason. If you have regeneration/ironskin/campfires/heart lanterns, this should not be too hard.
Once both twins are defeated, you'll get Souls of Sight and more Hallowed Bars to work on your armor set. See the main entry on Defeating the Mechanical Bosses to learn what changes in the game world based on your having defeated one or all three.
Souls of sight can be used to make a staff that summons a mini-set of twins to fight for you, and they do decent damage, at least enough to help you weaken a foe. They work wonderfully in large groups, provided you wear gear fit for a summoner (that which raises maximum minion count). You can make a Summoner Potion in 1.3 - see the combat potions page here on the guide for more info. The Witch Doctor also sells a bewitching table if you've also go the Wizard, which gives a 10 minute buff to raise minion count by 1. This is long enough for a boss fight.
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