Vanille

I have no idea, since I haven't beaten it yet, haha! I'm fairly close, though, but I've hit a bit of a dry spell recenently.
I'd ask Jeels or Shamdeo when they're online next.

Now, if you'll excuse me, I'm afraid I have to cut this conversation short; until next time, Vanille!

She also gains ATB segments more quickly than the other party members, which comes in handy as the chapters go on.
The little squeaks she makes when attacking can get pretty annoying as time goes on, though.

Exactly. His first form is also vulnerable to poison, which can do a fair bit of help in the long-term scheme.
Essentially, use Vanille as your party leader when facing it; she has both Poison and Death in her arsenal.