when an attack does more than 9999 damage.
In Final Fantasy XIII, there is a trophy to break the 99,999 damage limit. This can be done with Eidolons or by having a character equip a Genji Glove/Gauntlet.
do that wiht fang, she has the best strength. Either full strength mega flare, or equip gengi glaove and as powerful other equipment you can find on weak foe.
or, when you fight the proudclad the first time, stagger it, and use fang's full atb skill on it