The Ultima Weapon is obtained by synthesizing every single item, if you can't find a specific shard i'd recommend finding a list of where to obtain them (IGN and Gamefaqs are good for that stuff), after sythesizing everything you get a special title and Ultima weapon will either be unlocked or its the last thing to sythesize.
Ice Titan is all about timing, you just have to lock-on to the icicles he throws and then guard, they should bounce back and you will have obtained no damage. After bouncing them back a few times he will fall down and then you lock-on to his head and start thacking it. If you have curaga after a bit of practice it won't be to difficult.
Sephiroth is the most difficult villian to beat in the game (in my opinion) so he takes a bit more work, his health will go down after a little bit (as mentioned by grimmedslug) so you just have to keep hitting him, theres certain patterns that he uses. I haven't played KH1 in a while but if I recall using dodge roll is a really good way to dodge his attacks, when you go into the battle make sure you have plenty of elixers as his Descent Harvest Angel attach will drop you down to one (use it fast as he likes to jump in and attack you right after you're down to one) be wary of using to many attacks that use up MP as you will need to cast cure a lot. I was around level 79 or 80 when I beat him but others have said that they have done it around level 60 but i have no clue how they do that. just keep dodgeing and then when he stands still hit him, if theres any more to it, then i don't remember but i think that that should help.
