To increase your odds of making Death spell successful, enter the battle in the Espionage Paradigm, SAB/SAB/SYN. Fang makes the best second Saboteur, and either Sahz or Hope for the Synergist, though if you've unlocked the 10th level of the Crystarium, Lightning will also due. Every status effect Fang casts on the giant turtle will slightly increase the odds of Death working, and Vigilence will increase the odds of Death working for Vanille.

Also, Vanille should never attempt this while using any weapon but the Marboro Wand.