Razorblade Typhoon

Duke Fishron is the only source, and he never appears by himself. You hook him deliberately in the ocean using a Truffle Worm as bait, which means catching one of those first with a net in a mushroom biome. He hits harder than most things at his tier, so the usual pattern is to beat him with something else and then use this on everything afterwards. The drop is not guaranteed, so plan on summoning him more than once.

Each cast throws out a razor cloud that picks its own target inside a generous radius and chases it down. It pierces whatever it passes through, rebounds off walls instead of dying on them, and gains speed the longer it stays alive, so a shot that survives a couple of bounces will run down something quick that a straight line projectile would have sailed past. Liquids do not slow it either, which matters more than it sounds like it should given where you got it.

The homing is both the reason to use it and its one real flaw. Against the lunar events, a segmented boss, or anything that fills a screen, almost nothing is wasted and it competes with the best magic weapons of its tier. Against a single boss with minions around it, whole casts get spent on the minions instead. When damage has to land on one specific target, clear the adds first or bring something you can actually point.

Projectiles spawn a tile ahead of you, so pressing flat against a one block wall lets you fire straight through it while staying sealed inside. A small stone box becomes a firing position that almost nothing in an event like Martian Madness can answer, and you can clear the whole thing from inside it. Mythical is the modifier to hold out for, since you will be casting continuously and mana is the constraint rather than damage.