Blood Eel

A Blood Eel never wanders into you. It comes out of your fishing line. Cast during a Blood Moon in Hardmode and when the bobber dips, reeling in can produce an enemy near the bobber instead of a catch, and the eel is one of the things on that list. That makes it entirely opt in: if you are not fishing on that night you will never see one. A Sonar Potion tells you what is on the hook before you commit, so you can decide whether to pull or let it go, and a Chum Caster sharply raises how often Blood Moon enemies bite. Those two together are the farm, usually run for the Bloody Tear. A Lifeform Analyzer will also flag one, which is unusual for something that is not a rare spawn.

It flies rather than burrows, circling wide and then committing to a dive through you. The pattern will look familiar if you have fought a Wyvern, but the eel covers ground much faster, so the window between it lining up and it arriving is short. Fighting on a beach or a lake means open sky in every direction and no wall to be pinned against. The head is where the decision sits: it carries the least armor of any segment, so it is where your damage should go, and it also does the most on contact, so charging at it is a trade you lose. A Shield of Cthulhu dash timed as it commits, sideways or straight under the body, gets you out clean.

Anything that hits several segments in one go multiplies its damage against a worm this long. A repeater loaded with Jester's Arrows or Holy Arrows runs the length of the body, and thrown flails like the Sunfury or Dao of Pow do the same in melee range. Explosive rounds out of a Shotgun work. So does parking a Nimbus Rod cloud over your arena, or holding a Medusa Head or Crystal Vile Shard while it passes through you. The standout is Life Drain out of a Crimson Mimic, which pulls health from every segment it touches at once and turns the fight into a net gain. Its bestiary entry and banner also complete in half the usual number of kills, so a short farm finishes it.