Vanille's back.. this time to poison that annoying mark on the last mission. Unfortunately, he's cheesier than cheddar and uses that cheap impenetrable aura. So you have to keep recasting poison. Or do you?

If you enter gestalt mode, it stops him from doing so. You can drag this out and let the poison work. I killed off the last 1/3 of his HP (5 mill+ HP) this way. Vanille is ideal for this because she has poison, a good debuff weapon, and her summon is the only one with a gestalt move that expends one point. This allows you to really drag out the time and milk the poison and letting him have it.