I was wondering how to increase the success rate for Black Mages or Mystics when casting spells.
Having a caster with high Faith is essential to increase spell success rate. It seems to also be dependant on the MA stat, so anything that boosts magic and the Magic UP skill will help. (You can actually increase Faith/Bravery of characters, take a look at this)
It is also dependant on the TARGET's Faith/Magic evasion, Shell apparently helps evasion (as well as magic defense obviously).
Finally, it also depends on the Zodiac Factor (explained here)