If you have Alexander's Revive ability, you can kill Abadon in ONE HIT! You can also do this with a Pheonix Down, but it sometimes doesn't work. Abadon, being an undead monster (as your characters will state as you enter the battle), is of course, weak against healing attacks. If you don't have the Revive ability, all you need to do is throw a couple of X-Potions in his direction (usually about 3) and he'll be gone. Also, the Recover ability does some really great damage, about 9999 per hit. Try to avoid using Cure spells (Cure, Cura, Curaga), since they don't do as much damage, but if that's all you have, go ahead and use them, as well as Holy, and casting Alexander.