King Slime - Nimbus Rod, I always forget about him, and when he spawns, I'm already late to hardmode and used Nimbus rod for fun.
Eye of Cthulhu - Trident, Platinum Bow.
Brain of Cthulhu - Blood Butcherer, Blade of Grass, Tendon Bow, Diamond Staff, Water Bolt.
Eater of Worlds - Terra Blade, so far, out of tens of worlds I created, only two are corruption, and I fought them when end-gamed, except for my mage playthrough where I used Space Gun (heeehee, cheese)
Skeletron - Diamond Staff, Space Gun, Blade of Grass, Tendon Bow, Thorn Chakram.
Queen Bee - Aqua Scepter, Blade of Grass, Tendon Bow.
Wall of Flesh - Molten Fury, Blade of Grass, Night's Edge, Great Flamesword thing, Imp Staff, Space Gun, Flamelash.
Destroyer - Nimbus Rod, Poison Staff, Cursed Flames, Crystal Storm, Cutlass, Frostbrand, Bananarang, Titanium Repeater, Clockwork Assault Rifle, Optic Staff.
The Twins - Titanium Repeater, Megashark, Clockwork Assault Rifle, Poison Staff, Crystal Storm, Cursed Flames, Frostbrand, Bananarang, Light Disc, Imp Staff.
Skeletron Prime - Titanium Repeater, Clockwork Assault Rifle, Megashark, Nimbus Rod, Cursed Flames, Crystal Storm, Optic Staff.
Plantera - Unholy Trident (yes, I use them), Bananarang, Light Disc, Megashark, Optic Staff.
Golem - True Excalibur, Terra Blade, Flairon, Chain Gun, Inferno Fork, Razorpine, Tempest/Optic Staff
Duke Fisheon - Optic Staff (such a pain), True Excalbur, Terra Blade, Chain Gun, Razorpine, Possessed Hatchet, Paladin's Hammer, Vampire Knives.
Moon Lord - not available to us mobile players yet.
Ocram - True Excalibur, Cursed Flames, Terra Blade, Vampire Knives, Hallowed Repeater (pain in the rear)