I've been thinking a lot on how to beat Adel and i figured out 3 idealistic strategies:
1) Use Cerberus and then cast Float and Shell on everyone, than throw all the GF power on Adel and Rinoa and every once in a while use a Triple Curaga on Rinoa (normally after 2 GF attacks). BTW, u better have Cerberus with a decently big HP because Adel casts Meteor and Smashing Blow the first time at small intervals. Until you use a G-Returner it might be too late.
2) Everything from above, except use all the Aura on Squall (if you have a big strength) because I've noticed that Renzokuken + Rough Divide does more dmg than a GF with all the boosting. After you finish Aura use the GFs. The only difference is that Aura is 20% of the time ineffective.
3) Everything in 1) or 2), except cast shell on Rinoa and Adel shouldn't be able to draw much from her nor you do big dmg with the GFs on her anymore.
In case you're wondering: Float for Quake, Shell for Ultima, Flare, Meteor etc, Holy heals him; and these strategies are for hp-lowers (me: Squall-1916, Zell-1975, Quistis-3120), those with high stats might find this fight easier. One last thing, make sure that one of the characters has Item in case you run out of Curagas. And if any of you have any other suggestions please don't hesitate.