Toxophilite [East]
Terrarian
In not-so-many words, I'm trying to suggest playing the game with that intention in mind. You're likely to get several or more Blood Moons in pre-Hardmode, before Bloodnautilus becomes available. If you know you're gonna wanna farm for Sanguine Staff in Hardmode, you should plan for it in advance. Battle Potions are one of the easiest methods to doing this early-game & adding Water Candles later on is the most reliable. By the time you reach Hardmode, you should have about two stacks of 20, more than enough to make a concentrated effort to farm Bloodnautilus much later on.My issue isn't defeating dreadnautelus, my issue is that on my average playthrough I only get to fight 2-3 before running out of bloody tears.
Keep in mind that the stronger enemies in either tier have a higher Bloody Tear drop chance, so treat the Event appropriately & kill things quickly!
Sanguine Staff is definitely a post Mech item, but if the official item rarity somehow makes this untrue... I don't have an answer for that; considering the fact that Bloodnautilus is debatably tougher than every Mech, is an Event Boss, is rare & drops a rare, powerful item, says different. Perhaps it's ability to be cheesed was taken into account as well but, I won't continue to argue against solid evidence with my own personal conclusions.The sanguine staff has the same (color)rarity as other pre mech weapons... that is one of the most direct ways for devs to state at what point weapons are intended to be obtained. Nothing you get from the mechanical bosses helps with fishing or increasing the drop rate.