Guide to beating The Twins:

1. Go to the Underworld.

2. Mine up between 10-20 Hellstone.

3. Make a small Hellstone Bridge at about 200 feet or so (really can be wherever you want, but I don't suggest it being by your NPC's.

4. Craft a Mechanical Eye.

5. Equip Ranger Emblem, Cross Necklace, and Star Cloak. Also do two other accessories (If possible, all should have Warding, or Menacing)

6. Get a Megashark and about 1500 bullets (cursed, crystal, or meteor).

7. Summon The Twins, and destroy them.

How-to use/why it works: Stand on Hellstone, and when you take the damage from the Hellstone (which will be one damage), it will grant you a little while of invincibility. Just standing there will make you almost immune to the Twins attacks. Star Cloak helps to deal them damage when the melee you, but it is not needed. Ranger Emblem for more Megashark damage. You can also use a Sorcerer's Emblem and a Magic Weapon, but it is up to you,

Suggestion: Kill Spaz-a-whatever first. Then Ratz-a-whatever will be a piece of cake. Also, take a few Greater Health Pots with you, just in case (1 or 2 will do). You can't use an Obsidian Skull, or any other accessories that negate fire damage! If you do, it won't work.

Good Luck.