Help with a battle (spoilers...possibly)

[Carnage]

The best tactic in kingdom hearts is guerilla warfare. Hit run heal. Except for ice titan and easy people.

[Trumpet Thief]

Keep running away from Cloud, and attack Leon when you have the oppurtunity. After beating him, attck Cloud whenever he jumps, and sticks his sword into the ground, or when he just regularly jumps. After a while, he will fly around with one wing. Just try to run from him as much as you can, and try not to get hit.

[square_is_the_best]

So you need help with Sephiroth? Whenever he warps be prepared for sin harvest. He does it more often then not. There's a "cheat". Whenever he warps, fire a strike raid. It interrupts his sin harvest. Very helpful. Also you are invincible when you strike raid, so don't worry bout gettin hit. Continue the strike raid to judgement if you can. You should be able to beat him this way. Its probably a good idea not to try to counter his attacks as you'll almost always fail. And as always cure often and always have more health than sephiroth can deal in one health. Is sin harvest a one hit kill? Not in my book. Just use second chance. Of courseyou won't have any MP to heal so I suggest you use strike raid to stop it altogther. Anyway good luck with that.

[Lord Xehanort]

Sephy does a little pattern: Slash-Slash-Slash-Flare. If you atack him at all, he'll counter with Flare. Don't mess with any Combo Plus in this fight; if you hit Seph with more than a 3-hit combo, he'll vanish and rip you up from behind. You can use Guard during his Omnislash-like move. Counterattack works well in this fight, it works as an addition to Guard. Just use it to save yourself. Have Elixirs on-hand for your first few times. Also, if he teleports to use Sin Harvest, you can Superglide or Dodge Roll (a lot) to reach him and attack while he's saying his line. Doing so will cause him to trip over and stop the spell.

Good Luck!