Guide:The Twins strategies

Tactics
When fighting The Twins, it is perhaps best to focus on taking one down as quickly as possible rather than spreading damage between them. Each part has its own life gauge and eliminating one makes killing the other much easier.

Spazmatism is probably the easiest to kill first as it chases the player at the same level. Retinazer chases the player above the player, necessitating leading adjustments if using ranged weapons (which you'll probably use as both hit for significant damage in melee.) Just aim parallel to the ground and you can easily pump Spazmatism with arrows or shots.

However, even though Spazmatism is easier to hit initially, his second form may be harder to fight for some due to its constant chasing of the players with both it's flamethrower-like attack and its charging attack which is the most damaging move The Twins have. As such, choosing to eliminate Retinazer first may prove to be the more beneficial option as one may have an easier time avoiding Spazmatism's later attacks if Retinazer is already gone.

In either case, whichever a player decides to eliminate first, it pays to make sure damage is concentrated on one of the bosses as much as possible. Avoid doing damage to both to the point that both transform into their second phases. Killing one is much easier when only one is in it's second mode and the other is still in its first form. The first forms are more predictable and tame compared to the second forms.

A wide-open, flat area is recommended for the fight for ease of evasion. If the ground is level enough to run across, it's possible to just kite the bosses with bow-/gunfire by running and simply shooting them as they give chase. Mobility is key.

Note that once one of them is killed, regular mobs can now spawn during the fight with the other half.

Equipment
If possible, a Megashark with Crystal Bullets make this a much easier fight (along with a ranged-type armor set). You need to kill The Destroyer first to get the Megashark, however.

A Shotgun isn't advisable due to the refire rate being so slow and that to maximize damage, you need to be right next to enemies to ensure many bullet in the spread hit. A Clockwork Assault Rifle may work as it's comparable to the Megashark and possible to get sooner via Wall of Flesh farming. (You trade some power and overall damage over time for endurance, as you get 3 times the ammo usage, guaranteed. This compared to the Megashark slightly higher damage, faster firing rate, but only effective doubling of ammo with its 50% chance to not use ammo.)

Rocket Boots, Spectre Boots, Angel Wings and/or Demon Wings are recommended for increased mobility. If no Angel/Demon Wings are available, be sure to equip a Lucky Horseshoe or an Obsidian Horseshoe to negate fall damage from the heights you can reach. Or use a Featherfall Potion.

If farming the Wall of Flesh, a Ranger Emblem helps speed up the process of whittling away their health with its +15% ranged damage bonus.

If money is no problem and a player has rescued the Goblin Tinkerer, upgraded accessories and weapons can help immensely. Cost can be relatively cheap or expensive, depending on the item being upgraded and some luck. Increased movement speed or damage bonuses on accessories are perhaps the best. Speed eases evasion while damage allows faster kills. (Up to +20% bonus if all five accessories have bonuses of +4% of the same type.)