It seems to me that you heavily rely on GF's as your main source of attack. When i first bought the game that's the way i played it (such a young kid ), and had trouble defeating Adel too.

At the start of battle cast Shell & Haste on your party, Aura on Squall, & Regen on Rinoa. Count on your limit break's as your main source of attack, Squall's renzokuken the most notable, and just keep slugging away at her. Have one member of your team whose job is too solely heal (or if you really have trouble, have 2). Rearange the junctioning of Leviathan to the person who will be the healer to use Recover on your party (that's if you have the GF of course, Alexander also, for Revive). The battle should be won with that.

PS: Remember to Mug Adel to get a Samantha Soul.