Sand Elemental

You cannot summon her. She appears on her own during a Sandstorm in the desert, standing on sand of some variety, and the game wants real depth of it nearby rather than a thin cosmetic layer, so a sandy patch you laid yourself will not do. She is rare on top of that, which makes a Lifeform Analyzer genuinely useful: sandstorms end, and knowing whether she is around decides whether you keep hunting or go home.

She mostly just chases you, badly, on a wandering flight path that cannot cross blocks, and she speeds up as her health falls, so a long careful fight gets harder rather than safer. The damage comes from the tornadoes. Three of them drop in around wherever you are standing, at varying spacing, then sit exactly where they landed for a few seconds before fading. They do not track and they do not follow, which makes each one a puzzle with a known answer provided you move immediately instead of trading hits inside it.

Ancient Storms cannot reach through solid blocks, so a small platform of real blocks over your head switches the attack off entirely. The catch is that holding still in a sandstorm invites Dune Splicers and Sand Sharks up through the floor at you, and that trade is usually worse. Height works better. Wings, a horseshoe balloon or Rocket Boots let you rise clear of a tornado as it forms, and a rope strung well above the dunes keeps you over the ceiling the storms can reach while you shoot down at her. If the rest of the sandstorm roster is the real problem, drag her out of the desert and fight her somewhere quiet.

On Classic her body barely hurts compared to her tornadoes, so deliberately clipping her and riding the invincibility out through a storm is a legitimate way to cross one. Higher difficulties invert that completely and the habit will get you killed, so unlearn it before you step up a mode.

Every kill hands over a Forbidden Fragment, and those gate the Forbidden set, the Spirit Flame and a Sandstorm in a Bottle, so a short run of kills covers everything she exists to provide. She fights well above her official billing, in the same odd category as the Ice Golem and the biome mimics: boss-scale numbers, no boss health bar, a banner instead of a trophy, and nothing waiting for you on Master.