This article is from the Final-Fantasy and other Square Soft Games FAQ, by firstname.lastname@example.org (Nick Zitzmann) with numerous contributions by others.
The easiest way to defeat Adel is to use either Squall, Zell, or
Quistis. Simply cast Aura on them, which you should have drawn out
during the previous battle against Seifer, then use their best limit
breaks. One of Squall's Renzokuken/Lion Heart limit breaks will
instantly wipe out Adel. Also try Quistis's Shockwave Pulsar. Any move
with Zell will work, but avoid using Final Heaven (since it'll hurt
Rinoa as well) and Meteor Strike (which doesn't work during this
If that fails, or you don't have Squall's Lion Heart sword or Quistis's
Shockwave Pulsar spell, it'll be a little more difficult. Try these:
* Don't summon GFs, except for Cerberus and Carbuncle. Remember that GFs
will attack Rinoa, and you want to avoid harming Rinoa at all costs.
* If you have the Regen spell, cast it on Rinoa right away.
* Junction as much as you can to strength, vitality, and HP.
* Did you know that you can refine tents into Curaga spells?
* If you don't have Cerberus, then cast Double or Triple (preferrably
the latter) on someone who has lots of Curagas. That person will be your
designated healer for this battle.
* Keep fighting; Adel will drop sooner or later (and it's going to be
later unless you were able to pull off a Renzokuken/Lion Heart).
But wait! After defeating Adel, you'll be launched into a series of
battles that are even more difficult before you can go to the game's
final dungeon. If you are having troubles with Adel and want to cheat
your way around it, think twice.