This battle is a bit tough for those whom have characters with levels
below 40. Yeah.. The minimum level to battle Adel is at Level 55. I have
managed to scrap through with that Level. Note that you can't harm
Rinoa (but you can steal from her ^_^. I think her HP varies from
4000 to 9036. Remember that once Rinoa dies, you will get a game over.

First of all, let's focuses on Adel's arsenal of weaponry:
Drain - she will only drains HP from Rinoa [700+ HP each time]. Stock
and cast Regen on Rinoa [She has the Regen spell right?].
Magic (Meteor, Ultima)- A character with a Spr stats of 50 will meet
their doom in 2 turns. Just junction Curaga to boost up Spirit. Cast
Regen on the entire part and it will help. Ultima is rare used but
it can does around 2000+ to each member and affects the entire party.
Make sure that everyone's HP is above 3000+ at all times. She will
unleash strong magic after this phrase: "Magical Powers are
concentrated on Adel"
Energy Bomber - 50% of the damage based on Vit and the other 50% is
based on Spr. So its damage isn't that great compared to the Meteor
or Ultima.

The best party is the guy's team, [Quistis can also be a good option
her.]. Irvine Kinneas and Zell's Limit Break packs a punch! Remember
for those who are using Zell, do not use the My Final Heaven Limit
Break and those who are using Irvine, do not use Shotgun Ammo, Pulse
Ammo and Fire Ammo. All group attacks should be avoid, including GFs
and group-based magic such as Ultima, Meteor, Quake, Tornado, etc.
Holy magic is highly recommended. Why don't use Squall? There are
about 55% chances that Squall will use the Blasting Zone and Fated
Circle in his finishing blow. His Limit Breaks are too damaging
somehow. This is the time when I found him least useful in combat.

(from http://db.gamefaqs.com/console/psx/f...i_na_b_1_2.txt)