Skeletron
Skeletron is the Dungeon Guardian that instantly kills you the moment you get within a set depth in the dungeon. To travel safely in the dungeon, Skeletron must be defeated.
You may have noticed an Old Man in the dungeons entrance. Speaking to him at night and selecting the only option will cause Skeletron to spawn. Skeletron is a floating skull with arms. It uses its arms as its primary way of attacking.
IMPORTANT! Skeletron can only be summoned at night and must also be defeated that same night. Going over this time limit will cause him to retreat and despawn like the Eye of Cthulhu.
Skeletron doesn't need a fancy Arena. Just a long flat area where you can run unhindered. Make sure the area is also well-lit. Since you can only battle it at night and having light around may help you in seeing if there are other enemies that are heading your way.
The fight itself is pretty simple. All you need to look out for are its hands and its head. The arms, despite looking to take up the most space, are completely harmless and you can go through them. The hands however are the main concern since Skeletron flings them around so much.
This fight is all about your jump timing. When you get the timing of its hand swipes, you've got this fight nailed down. It will also spin its head while charging at you. This does the most damage among its attacks. However, it is also the moment where it negates its defense. Hitting the head while it spins will deal the most damage. Rinse and repeat the strategy and Skeletron will be no more.