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.)

dart193's strategy
Build a platfrom 1000 blocks long(4 stacks of dirt is what you need). Gather adamantite armor. Have as much defense in accessoires as possbie.

Now the most important - potions.

1. Ironskin potion. The more defence the better.

2. Swiftness potion. Highly recommended, a must if no hermets boots.

3. Regeneration potion. Lasers and cursed flames are not that hard to avoid, but when Retinazer enter second phase, you will receive a lot of damage, so your health must be replenished.

4. Gravity potion. Spazmatizm will move much faster than you when charging at you in second phase. It renders almost impossible to avoid him without flying. However, if you have wings, then this potion is not that useful.

A weapon I choosed was megashark and crystal bullets. You will be running a lot and avoiding shots, so no time for aiming. Crystal shot will deal massive damage as when the eye charges, it "collects" every shard

The battle
Start running as soon as possible. The run left, right, left right on your platform. This will negate their charges as they are just slightly faster than you so you should be far away when they charges at you. Concentrate a fire on Retinazer. There are two reasons: first one is that his lazers are harder to avoid than flames, second - the second phase of Spazmatizm is dodging most the time so you will want to have less obstacles including the Retinazer.

Killing Retinazer is not hard, even when he goes second phase. Just jump a lot or fly.

When done with Reti, shoot Spazmatizm. When he goes second phase... well, you die, if you are not prepared. He will do a line of flames so long that you have to keep it almost out of screen. When he charges, he is ... 3-4 times faster than you. So you have to fly or jump very much. BUT when he flamethrowing, he is slower than you, so the strategy is: shoot him when he shoot you, avoid him when he is charging you.

The battle is not that hard even with not big defence, you only need to fly a lot.

Small advice: if you are running low on HP, try to get to the Nurse and heal. This saved me a iot.