[Hydra]

Don't use aeons on her, unless it's SURE to kill her in her final form, or else she'll just Curse and Absorb. Cast Hastega with Tidus, then cast Reflect with Yuna (And if you have Holy, use it. :O). Reflect everyone, then go with an all in attack with Tidus and Auron. The "Blind" will bounce off with reflect.

When she switches to the second form and uses "Hell Biter", make sure you still have reflect casted. She can't really hurt you again, unless she uses another Hell Biter attack. Make sure you can attack more than she heals each turn. If you can, you'll breeze through this form also.

Lastly, she'll open up with "Mega Death", which kills every "non zombie" on the field. Luckily, since you never bothered to cure it(did you?! :O), it doesn't affect you. She gains a new attack called Mind Blast, which can and will screw you over if you don't cure up afterwards. For the person who isn't under confusion, heal everyone with Remedies, and keep going. She will use Hell Biter again, just in case you want to take someone out of Zombie, but she will also use Mega Death again.

I had an instance where Mind Blast killed two out of three of my people (Yuna and Tidus). Auron used a Mega Pheniox to sacrifice himself to heal the other two. She reacted with Mega Death, and killed them both. Since the pheniox effect killed Auron in the process, I got a Game Over. XD