Abadon
Abadon is one of those rare, undead creatures and you know what that means expereinced FF gamers; yep it can be killed instantly with an elixir, a phoenix down, or Alexander's Revive ability. However, assuming you do not want to take the cheap way out of this battle we will continue with how to take out this massive creature.
While the listing says that Abadon is weak versus fire and holy neither will do as much damage as the Recover skill will, which will hit for 9999 damage. A less cheap while still being cheap way to kill it. Abadon has two forms, his first one lacks defensive power but has strong attacks while the second is strong in both aspects of defense; physical and magical, and will focus on bludgeoning your party with magic..
Abadon's claws can cause a multitude of status effects so be prepared to heal your party memebers if they become afflicted with something. Treatment can be a nice ability to have because of this, but if you do not you can draw esunas from the boss and heal that way.
Do not use cure or curaga on this boss as their effects will be minimal at best, but if that is all you have you might as well use those as well. Abadon has a good chunk of HP so it will take some time to bring him down if you are going with the legit method. Remember to keep your health up and to heal status effects as they are applied and you sohuld be able to defeat this boss easily.