After a boss run with only the Zweihander, I find it an interesting novelty weapon. That said, the hitbox on the Possessed Knight is seriously off considering its size.
It's just me or Candore is still the best boss in the bunch? Variety of attacks and phases, plus it ain't your stock monster. The others show creativity (bar Possessed Knight) but they are considerably less fun to fight. Giant Spider comes second, even if it's trivially easy to cheese, while Giant First Boss and the Knight suffer from... both lack of difficulty and interesting patterns.
If something needs a rework, it's indeed the Knight: it's almost easier to fight than the First Giant Boss, and it's predictable and slow. Faster attacks? More punishing specials? Whatever, it's not like the dev reads this forum.