First of all... are you mad? How did you managed to get to the 4th disc with only 25 level? Anyway, I recommend you to level up and I guess you got the GF Leviathan, right?
If you do, then fight until you have mastered the Recover ability (see the GF menu), once you do that and have equipped that hability, do this:
1- cast shell and protect over Rinoa
2- cast meltdown on Adel
3- attack Adel with Irvine´s limit, I highly recommend the Armor shot, because it damages more than the others, meanwhile, with another party memeber summmon a GF (ifrit, shiva or quetzacoatl) and finally, with the other party member use the ability Recover over Rinoa. This ability recover 9999 HP....

Hope this makes your life easier. If you have another question don´t hesitate and ask me, OK? my email is TeriYaqui12@hotmail.com