Piranha Gun

It is not a drop and it is not crafted. It sits in the Jungle Chest inside the dungeon, which takes a Jungle Key and refuses to open until Plantera is dead. The weapon therefore arrives at a fixed point in progression, and there is nothing to farm for it beyond the key.

Hold the button and it throws a spread of mechanical fish that fly out, bury themselves in the first thing they touch and start chewing. From that point you are not aiming any more. They ride the target through its own movement, jump to whatever is nearest when it dies, and come home when you release. Anything standing right beside the victim is caught in the same bite, since each fish damages a small square rather than a single point. Keep the button down and returning fish are flung straight back out toward your cursor, so the stream does not really stop until the timer runs out.

Each fish has a distance budget. One that flies off without finding anything turns around fairly quickly. One that is latched on, or chasing a replacement target, will follow much further. Past a hard limit it simply vanishes. It also needs a clear line to a target before it will pick one, but once it has committed it travels through terrain to stay attached. That distinction is worth internalizing, because it decides whether a wall is helping you or getting in your way.

Attached fish move through walls. A teleporting enemy cannot shake them, and a Paladin that never gets a look at you never throws anything back. Stand behind a block, hold the button, and rooms that would otherwise cost you most of a health bar clear themselves. As an answer to the post Plantera dungeon it is close to the best thing available at that point.

Whatever bonuses applied the instant a fish left the gun stay with it until it comes back. Firing out of a ranged stealth state keeps the extra damage running long after the stealth has dropped, which is a real trick worth using, though the critical strike half of that bonus falls off immediately. The flip side is that nothing you do mid fight improves the fish already in the air. It also picks up the Shroomite Helmet bonus despite firing no ammunition at all.

Because aim stops mattering after the throw, this is one of very few weapons that lets you spend an entire boss fight thinking about nothing but movement. It keeps damage on the target while you dodge and wait out potion sickness. It holds on through certain invulnerable phases as well, so a boss that normally shrugs everything off during a transition keeps taking hits. Costing no ammunition is what makes absurd grinds like the Dungeon Guardian worth attempting at all: slow, but free.

Unreal is the nominal best, but attack speed does nothing here, so godly and demonic land in the same place. A velocity prefix only affects the initial throw, not the chase or the return trip. Two other quirks: the fish move slower through liquids, which is faintly ridiculous given what they are, and they refuse to latch onto other players, so it is dead weight in duels.