Sea Snail

Nothing else in the ocean spawns as rarely, roughly a third as often as the Squid you see constantly, and it lives down on the seabed, so the search happens underwater with a breath meter draining. That is the actual obstacle. A Diving Helmet, or anything that removes the breath limit outright, turns the hunt from a series of panicked trips to the surface into something you can sit through. A Lifeform Analyzer will tell you when one is nearby, which beats every other trick here.

A Water Candle and a Battle Potion work on it the way they work on anything rare. The trick worth knowing is the solar eclipse. It pushes spawn rates to nearly blood moon levels, and eclipse enemies themselves cannot appear in water, so they end up stranded on the beach or in caves below the seabed and rarely come out to bother you. The result is a large spawn boost applied almost entirely to the normal ocean roster, which is exactly what you want when your target is part of that roster.

It crawls along the bottom and hurts you by touching you. No projectile, no pattern, nothing to learn. The only real risk is drowning while you deal with it, or being caught underwater by something faster that turned up because of whatever you did to raise the spawn rate.

Purple Mucus does one thing, which is Purple Dye at a Dye Vat. If you are not dyeing anything, there is no reason to run this hunt at all. Real purple dye was made from sea snails and cost about its own weight in gold, which is a large part of why almost no national flag uses the color.