Hardest boss for me?
Low level Sirius. Endrance is the only one who can survive more than 3 attacks, while Haseo and Kuhn die by simply Sirius's shockwave, making it impossible to attack him with anything but skills. And you shouldn't ever use a skill when it's not against Sirius or with Sirius able to use a skill, because he'll Hangeki you to death, and usually the Hangeki will hit you even if you're close to the target thanks to an evil lag (happens very often during the first skills you use, and usually when it's not a weapon change skill). And there's also Balmung and Orca to deal with, that while you shouldn't bother fighting them, they'll still get in the way and sometimes attack very often, and you HAVE to counter every skill they use if it's against you; weapon change invincibility is not enough. And on top of that, you can't use an Awakening in the first half, and you have to kill him on the second part with only one Awakening, else Demise Claw will kill you. You can actually get killed by it and win the battle, but it's a random wierd thing and you have to decrease his HP to 300 anyhow >3>''
And low level Taihaku without using any juggling strategy was so darn hard too, but i guess it's just plain impossible to beat him at a low level without skills like Tiger Strike XD