I thought that Ice Titan was quite a pain, to be honest. Not in the difficulty of it, because it was quite easy. I just mean about the rate at which his health goes down. It's ridiculously slow! I remember that at first I couldn't work out what to do but a few minutes later I noticed the ice shots were damaging him so I went on with that. It was annoying how I kept missing the ice shots by either a second too early or a second too late. But yes, he was an easy enough boss--I think the problem is that you have to have patience with him.
Phantom, for me, was difficult at first, because I was only in the levels of the high-forties, low-fifties, so I trained up more until around level seventy or eighty, I think, and then had another go, and it was much easier. The problem, for me, was the clock. It took me a few moments to realise what was going on.
Kurt Zisa was a pain. I hated him. ABSOLUTELY DESPISED HIM. But I trained to level 100, made sure I had good defence and attack and Ultima Weapon and good items, and then went against him and only just won. I think he was annoying me so I panicked a lot. But yes, he is generally quite difficult. His name is cool, though.
Sephiroth for me, at first, was impossible. I first tried defeating him in the high-forties, and then I realised it was impossible so I went and trained to level 100, made sure I had all Elixirs, great defence and attack, and disabled all the 'limit' moves apart from Ars Arcanum, and made sure I had Ultima Weapon. I defeated him quite nicely after that. When I fight him these days, I find him quite easy. My record is 2:01, but only because of an insanely good Ars Arcanum shot, and also because I had a lot of luck with guessing where he was going to land and hitting him, and not getting burnt by the fire pillar attack.
Good luck!