As you're just about to enter the Farplane I am assuming you already know how to use the Customise option. Using Ability Spheres (either 2 or 4, I can't quite remember) make a weapon have Distill Ability. This way you will always receive Ability Spheres when the enemy is defeated. Another was is to purchase the Ability Distiller item from the item shop and use it on the enemy. This has the same effect as them being hit with a weapon equipped with Distll Ability. If you land an Overkill then you will get twice the number of spheres.