Toxikarp

The bubbles go where you point for a short stretch and then climb. That single behavior decides everything about how you use it. Enemies at your level or above are easy targets, anything below you is nearly impossible to hit, and the practical technique is to fire slightly under a target and let the rise carry the shot into it.

Bubbles pass through platforms without popping and start climbing immediately on contact with water, so layered arenas and flooded caves suit it. Everything it hits is left poisoned, which does more work than the direct damage suggests over a long fight.

It costs no ammunition and no mana. In early Hardmode, when you are burning through arrows faster than you can craft them, a weapon that fires forever is worth carrying even after its raw numbers stop looking impressive. It is the Corruption member of a set of four fished weapons, alongside the Bladetongue, the Crystal Serpent and the Barnacle Staff.

The catch rate is brutal and you will pull up a lot of junk before it appears. Keep the Ebonkoi instead of tossing them: they brew into Wrath Potions, which turns a long fruitless session into something you can still use.