This is just an opinion of mine but the Dark knight dressphere paired with the White Mage is somewhat an overstatement in terms of power. Since Darkness can heavily damage all enemies and consumes only a very small portion of the Dark Knight's HP (provided that DKs have the highest HP of all desspheres) followed by a White Mage who recovers the lost HP afterwards. I obliterated almost all bosses this way and I didn't give too much sweat.