In the game of Terraria one of the main bosses is Skeletron.
When defeating Skeletron I used silver armor, gold sword, demon bow,
war axe of the night, a few lesser healing potions, and had about ten hearts.
He was fairly easy to defeat after some practice.
I’d recommend killing the Eye of Cthulhu a few times first.
Maby even Eater of Worlds too.
Make demon weapons and maby ever some armor.
Try going outside/on top of the castle to get a nice open area to fight.
Also try using mostly your bow until a spin attack, then use a fast melee weapon.
Hope this helps 🙂