This is a tough boss to kill due to its high HP and defenses. Imperil works, but it resists other status ailments, especially Poison. It might be worth leaving Vanille as a Saboteur anyway, since Deprotect and Deshell can shorten the battle. However, Sazh's SynergistAbilities are much more reliable, especially Haste and Enfire or Enfrost. The boss likes to inflict Poison, which can be healed with an Antidote or Vanille casting Esuna. Once it releases its "arms restraints," watch out and keep your HP up.