The Destroyer is a hardmode boss.If you don't have proper preparation, he will easily defeat you.To learn how to kill him, begin with step one.
Step 1: Understand who Destroyer is.
He is a boss similar to Eater of Worlds.He is taller and thicker.Each unit of his body has a red flashing light, which eventually detaches and becomes a probe that fires lasers at you.He drops some Hallowed Bars after he's defeated.
Step 2: It's a good idea to gather equipment.
If you are able to farm Frost core/Forbidden Fragment, you can get Titanium armor/Adamantite armor with helmet.You will need at least 12 greater healing Potions, which can be made from 12 bottles of water, 12 Pixie Dust, and 4 Crystal shards.
Step 3: Get a weapon.
If you are a melee user, you could try Adamantite Glaive/Titanium Tridents.Depending on your world's evil, ranged users can use a variety of weapons.Most magic weapons should work for magic users.
Step 4: Give him something to do.
You need to make a mechanical worm from rotten chunks, iron bars, copper bars and 7 souls of night.At night, he can only be spawned.
Step 5: Remove him.
He leaves the screen so be ready.Destroyer likes to go underground and zip up at you.Attack him until he dies.If you have low health, drink a Greater Healing Potion.If your health gets really low and the cool-down for healing Potion isn't up yet, then you can go to the nurse and get Magic Mirror/Recall Potion.He might kill a few of your people.
Step 6: It was done!
He drops Souls of Might, which can be used to craft Excalibur, Megashark, and other useful items after he's dead.