Guide:The Twins strategies

The Twins are the hardmode version of the Eye of Cthulhu. They are the mid-boss of the hardmode bosses, stronger than The Destroyer but weaker than Skeletron Prime.

Terrain preparation
Space is a must when battling the Twins, since they share the same attack patterns as the Eye of Cthulhu, albeit slightly modified. Several layer of wood platforms, at least a full stack or two long, will aid greatly in avoiding their charging and projectile attacks. A solid ceiling will block Retinazer's beam attacks. Another good way to battle the Twins is to create a huge sky bridge (roughly 500 blocks long), equip Hermes Boots or Spectre Boots then run along it, shooting the Twins with your weapon. This is a good way to dodge the Twins during their first phases (and Spazmatism's second phase if you have a grappling hook or Wings) but you will need wood platforms to dodge Retinazer's second phase and Spazmatism's second phase to some extent.

Armor
Adamantite armor should be used if the player has not collected the souls necessary for Hallowed armor. The ranged set bonus should be used, since dodging the Twins is easier in the first phase of the battle, and the damage boost is helpful in eliminating the component parts faster.

Weapons
The Adamantite Repeater with Cursed Arrows is the optimal weapon for players who have yet to beat any hardmode boss. The Hallowed Repeater or Megashark with Crystal Bullets or Cursed Bullets present a better alternative if either The Twins or The Destroyer have been defeated already. Other weapons that work quite well are the Clockwork Assault Rifle (with Cursed or Crystal Bullets) because it has reasonable damage and a good range. The Star Cannon is also useful if you have enough ammo saved up.

Accessories
Accessories that improve mobility are a must, since the Twins use quick attacks. The Spectre Boots and either the Angel Wings or Demon Wings will allow the player to dodge their charges and escape their ranged attacks. The Obsidian Shield will prevent knockback. The final accessory should help in preventing direct damage; the Philosopher's Stone, Band of Regeneration, or Cross Necklace are all good choices. All accessories should be reforged for damage or defense bonuses, at least +2 for each.

Potions
Bring plenty of Greater Healing Potions, at least ten to be on the safe side. The Regeneration Potion and the Ironskin Potion will help prevent fatal damage, but will often wear out before the battle is over. Two of each will cover the entire night. A Bowl of Soup will provide a further advantage. The Gravitation Potion is especially useful, as it eliminates the need for any arena and may free up an accessory slot. (Keep the wings, to prevent fall damage, but if you stay in the air you don't need the spectre boots.) Combined with the Spectre Boots and Wings, the player's arena effectively encompasses the entire map, and allows you to circle around the Twins to more easily avoid their attacks. The last potion that will be of great use is the Swiftness Potion, because it will help you evade the twins.

The battle
The Twins, as the name suggest, are two separate monsters, both of which must be killed. They have a combined health of 48,000, shared equally between them. Retinazer, distinguished by its red pupil, fires lasers. Spazmatism, distinguished by its green pupil, fires cursed flames. When reduced to half health, they will transform, gaining a slight boost in offense and defense. They will not both transform at the same time; both must be reduced to half health for both to transform. When one is destroyed, the other will continue attacking in the same pattern.

Retinazer
In its first form, Retinazer will fire lasers at the player at a steady but somewhat slow rate. This is occasionally interrupted by charging attacks, which mirror that of its partner. Retinazer does slightly less damage than Spazmatism, but this is offset by its higher accuracy thanks to the speed of the lasers. When not charging, Retinazer will always be aligned vertically with the player, positioned above them, which makes its shots easy to block with a solid ceiling.

When it changes into its second form, Retinazer will no longer engage in charge attacks. Instead, its lasers will fire with increasing frequency as it loses health. This is occasionally supplemented by a weaker but much more rapid laser attack that tracks extremely well. To avoid this, try to fall as fast as you can (Preferably without fall damage), and you can be left unscratched. With wings it is possible to "zig-zag" up and down to dodge the lasers. This is made trickier as the fight goes on, with Retinazer using more rapid and fast lasers that are therefore harder to dodge.

Spazmatism
Spazmatism, like Retinazer, varies between firing cursed flames from a distance and engaging in charging attacks. The flames do more damage than the lasers, but aren't as fast and thus easier to dodge. Spazmatism is always aligned horizontally with the player.

Spazmatism's second form changes his attack pattern dramatically compared to Retinazer. Its cursed flame attack becomes a constant stream of flame, which will last for several seconds. However, it doesn't chase the player quite as quickly as it does in its first form, making avoiding the attack somewhat easier. Once it stops spewing flame, it will charge with greatly increased speed, which also does a lot more damage. Dodging these charges is quite difficult. Much like Retinazer, fall as fast as you can while he is charging, and you should be okay. Misdirecting Spazmatism's charges is easier than it sounds; if you jump as he slows down, he will charge over you. This causes Spazmatism to charge at you from either above or below, making him easier to dodge if you just run.

Which to kill first?
Though killing one or the other first is largely a matter of preference, one should always be killed before causing the other to transform. Fighting both in their second state is far more difficult. Spazmatism can be considered more of a threat in its second state, which may make taking it down first advisable since Retinazer will be easier to avoid without interference. On the other hand, Retinazer's precision attacks may make it worth prioritizing over Spazmatism, which will have a harder time hitting a mobile player.