Werewolf

Full moon, after dark, in a Hardmode world. That is the entire spawn condition and nothing you own can force it, so hunting one is a scheduling problem before it is a combat problem. A Sextant reads the phase straight off. Without one, the shops tell you: the Painter rotates his paintings with the moon, and the Clothier, the Dye Trader, the Zoologist and the Skeleton Merchant all swap stock on particular nights. The Zoologist is the easiest read of the lot, since she visibly changes on the night itself. A Blood Moon does not help on its own either. Werewolves only join one if the moon happens to be full as well.

They lunge rather than walk, so the gap you thought you had between you is not there. They also open doors, which quietly removes the standard early answer of shutting yourself inside a house and waiting for morning. On top of that they can leave you bleeding, which shuts off your regeneration at exactly the moment you were counting on it. If you are underequipped for the night, wall yourself in with real blocks rather than a door and leave a slit to shoot through, the same as you would for a rough Blood Moon.

Silver Bullets hit them for triple, which is the old folklore wired into the game. It is the only place in Terraria where silver does anything special, so it is worth loading a stack for the night and going back to whatever you normally shoot afterward. No silver armor or silver weapon does anything similar.

The reason to hunt them at all is the Moon Charm, which turns you into one at night and feeds into the Moon Shell and eventually the Celestial Shell, an accessory you will keep for the rest of the game. Because the window is a single night at a time, push the spawn rate hard while you have it: a Water Candle and a Battle Potion together make a real difference over one night. Wearing the charm yourself buys you no peace, incidentally. They attack you in wolf form exactly the same.