Ice Golem

There is no summon item. It only spawns in the snow biome while a blizzard is running, it is rare even then, and only one can exist in the world at a time. It also despawns the moment it slips off your screen, so once you have found one you have to stay near it. A Lifeform Analyzer will call it out, but flying laps of the snow and watching for beams in the dark finds them faster in practice. If you want a lot of them, there is a world seed that drops you into a Hardmode world that is entirely snow and permanently storming, which turns a weather hunt into a farm.

It walks slowly and will never catch you, so the whole encounter comes down to one shot. Given a clear line it fires a fast beam that chills you and can freeze you outright, and being frozen next to it is how this goes wrong. The gap between shots is random rather than fixed, and the entire range shrinks as its health falls, so a drawn out fight gets more dangerous instead of easier.

• It needs a direct line to you. Break that with terrain and the shot is wasted and goes back on cooldown.
• It cannot fire while it has any vertical movement, so it is harmless mid jump and mid fall.
• It will not fire while you are already frozen, which is the one rule stopping a freeze from chaining straight into another beam.
• Every hit it takes can push the timer back a little further.

Constant damage stops the beam from ever leaving, even with you standing in plain sight. Anything with a high rate of fire holds that lock, and an early Hardmode summon holds it while you handle other things. Once it is cornered a yoyo finishes it comfortably, and the Hel-Fire adds a burn on top. The problem is the weather you needed to find it: a blizzard spawns plenty of other enemies, and one interruption is all the golem needs to get a shot away.

The Frost Core is guaranteed, and that single drop gates the Frost armor set, the Cool Whip and the Blizzard in a Bottle. Its bestiary entry and banner also unlock after fewer kills than a normal enemy asks for. For something that fights like a boss, the game files it as ordinary: no boss health bar, a banner rather than a trophy, and nothing extra waiting in Master Mode.