itz easy..1st u need cactuar gf and learn kamikaze..use kamikaze to adel.she will dead in no time

First of all, bigboi5 is tlking about Edea on disc one, and second you should ignore him.
Here's what the different characters should do:
Squall: Keep attacking DON'T use Renzokuken as two of the finishers attack both people.
Zell: Have Aura cast on him then keep using Duel but only use combos that attacks one enemy.
Irvine: Have Aura cast on him then keep using Shot but only use ammo that attacks one enemy.
Quistis: Have Aura cast on her then keep using Blue Magic but only use magic that attacks one enemy.
Selphie: Keep attacking with strong magic and possibly using status boosting magic on your party.

DO NOT: Summon a GF that attacks all enemies or randomizes the enemy. Use magic that damages all enemies.

DO: Keep healing Rinoa and follow the guide i gave above.

Have all characters with life of 3000 at least. Equip Magic, Draw, Item and your usual Attack.

With Adel cast haste, shell, regen and protect on all of your characters. Then dont use gf's or they will kill Rinoa in 2 hits or more. Cast regen on Rinoa and hit Adel with magic like meltdown and bio. And use Meteor only a few times but fill Rinoa up after each go. If you have Odin he will most definatley appear at the end of the battle and attack both Adel and Rinoa. But Adel will die first.

When I did fought adel the first time I thought she was absolutely impossible to beat so I started looking for cheats.

In all the cheats I found people said to NOT use GFs and only atack with limit breaks. I tried but she kept healing herself and my attacks weren't very strong (300-400) and I had to keep healing myself coz of her attacks and then she healed herself with Rinoa and would never die.

so I decided the only way to beat her (for me-should work for u) is to use GFs. My party was Squall, Zell and Irvine (doesn't really matter). I junctioned most GFs to Squall and made him in charge of attacking (use another character if compatability is low). I gave Leviathan and Alexander to Irvine, giving him the abilities Recover and Revive and he was incharge of healing (with Recover) Rinoa every turn (or party if Rinoa's health was already full). Zell had a few GFs junctioned and basically healed party and attacked.

I found Pandemona worked very well, don't use Alexander it heals her. I hope this helps. remember Rinoa is your priority over party. I tried regen on Rinoa to but it didn't last long and one more thing I was at lvl 32 with Squall and lvl 21 with the others, the weaker you r the weaker adel is. Make sure your GFs (such as Brothers, Carbuncle etc) learned HP 80%, or HP 40% it helps heaps.

Train up until you are at lvl 100 with Squall at least junction magic and abilities to raise strength to 255, when it begins cast aura on Squall and use Renzokuken the higher his hp the less chance that he'll use a finisher. Adel will die with one aplication.

To defeat Adel, when you fight her you must use your GF's every attack, use Shiva mostly, whne you get close to defeating her she will kill Squall and you advance to disc 2.