Defeated the Hand of the King on 2 BC / New Game++, earning me the ability to turn up the difficulty one more notch. (Some new, harder monsters; and the game's gotten downright stingy on between-level healing.)
I was having an excellent run, blasting through the levels taking only minimal damage; such that I was able to preserve my healing potions. Then I stumbled onto some really extraordinary luck - on one of the final levels leading up to boss (so when item levels are their highest, so weapons will have lots of affixes and high base damage), I came across a legendary weapon shrine. Defeating its guardians, I was awarded with a legendary-grade Starfury (a Terraria crossover weapon).
The regular Starfury is decent, but nothing special - its high rate of attack is balanced against the fact that it's not very good against normal enemies, since its attack doesn't cancel their attack animations. Each time you hit an enemy, two stars are spawned, which deal critical damage to random targets on screen - not so great when there's an entire screen of things that the stars might target; quite deadly when there's only a single enemy (like a boss battle.) Legendary Starfury doubles the number of stars spawned with each hit, more than doubling its damage.
It was still a difficult fight - if I hadn't had nearly all my healing charges I might still have lost - I actually chugged one potion while I was standing in a spike pit, since the pit was slightly safer than possibly getting juggled by the boss attacks.
Edit: Wow, and just a single day later, I managed to beat the game on 3 BC / New Game+++ mode.
Turns out that having a heavy hitting, fast weapon that deals criticals on enemies who are held (Baseball Bat) and a thrown weapon that inflicts a long-lasting hold effect - even to most bosses - (Boy's Axe) makes for a formidable combo. I even managed to finally scoop up a 'no hit' achievement on one of the bosses that I'd been unable to perfect before now.