Oh, it's been so long since I've fought Sephiroth. Alright, here are the strategies I used:

Sephiroth:
Keep your distance. He'll eventually jump high up and land near you. He'll be totally vulnerable to a few good blows. He's also quite vulnerable right after he casts flare. You must be quick, though, if you're going to attack him after he casts flare. Once you get him to a low enough HP, he'll start to use Sin Harvest which takes away all but one HP and MP, I believe. Fear not, though. He usually teleports near you, so you can easily reach him and attack him. If he's halfway across the arena from you, use Strike Raid on him. If you're timing's right, you'll be able to hit him at least 5 times with Strike Raid. Sephiroth also does Omnislash during this form. Just cast Aero and dodge and you should be fine. His third form is probably the most annoying. He constantly teleports around the ring and he uses meteor. Make sure to always have Aero on. Don't bother attacking him while he casts meteor, though, because he's invincible then. After meteor, he will be vulnerable, so just keep slashing away. He'll constantly teleport behind you and attack, so watch out, keep Aero on at all times, and slash away until all his HP is gone.

Ice Titan:
When he throws those icicles at you, just block them and they'll hit the Titan. Keep doing this, and he'll faint. Once he does that, use Ragnarok or just normal attacks on him. He'll eventually get back up and keep doing the attack. He'll start to make large icicles rise out of the ground to hit you, once his HP's low. Keep dodge-rolling until this stops. Once his HP's very low, he'll start using his freeze breath, which will freeze you in your tracks. His icicles will also get bigger and stronger. Remember not to use Aero. No matter what, never use Aero.