Amarok

It comes off any enemy in a snow or ice biome, including the corrupt, crimson and hallowed versions, so the fastest route is whatever produces the most bodies per minute rather than whatever is toughest. Statue farms are out. The roll only happens on enemies that also drop coins, and statue spawns never do.

The two practical setups are King Slime, whose minions each roll separately and who costs almost nothing to resummon by Hardmode, and a goblin army caught in a snow biome. Both flood the screen with weak targets, which is what a drop this rare actually needs.

It is a clean step up the yoyo ladder for early Hardmode, and the chill it applies keeps working while the yoyo sits parked on a target. Yoyos want a counterweight and a glove before they feel finished, so pick those up alongside it rather than judging the weapon bare.

In Inuit stories the amarok is a giant wolf spirit that hunts alone and preys on anyone reckless enough to do the same after dark.