Just finished mission 64 with total 7-8 tries yesterday. Something to share here:
My 3 chars all fully leveled, equipped with ultimate weapons, party members: Fang/Lightning/Hope
Equipped Fang and Lightning with Genji Glove, useful for COM/COM/RAV paradigm after staggered. Also equipped with Gold Watch if want to hit 5 stars.
Use Aegisol and Fortisol before start the batter.
Paradigm:
1. SEN/SEN/SEN - to minimize the damage of Whicked Whirl
2. SAB/SAB/SAB - prepared to debuff it especially after he cured and buffed itself.
3. SEN/SYN/SYN - Fang SYN ability a bit different with Hope and Lightning, better use her as SEN or maybe SAB.
4. RAV/RAV/RAV - to boost the chain fastest
5. COM/COM/RAV - to hit hard it when staggered
6. MED/MED/MED - To heal your party as fast as possible when necessarily
At the beginning, it has a routine of attack, if you found out it's pattern, then is very easy to kill it.
Start out with all the shrouds, and then go in with SAB/SAB/SAB, debuff it for 1 time, then quickly change to RAV/RAV/RAV to try to stagger it, if you lucky, maybe able to stagger it and kill a number of HP, because it will go into Impenetrable Aura very fast. Then cure your party to full HP, change to SAB/SAB/SAB, and prepare Fang with DEPROTECT/DESHELL (3 ATB), and SLOW (3 ATB), leave the DISPEL job to Hope and Lightning. Give another fast round of debuffed, probably if you saw Deprotect and Deshell and Slow ON, change to RAV/RAV/RAV again, once staggered go for COM/COM/RAV until it Impenetrable Aura, this might continue for around 3-4 rounds before it gone mad with more fierce and fast attack.
Then when it didn't go to Impenetrable Aura too often and attacked longer (with Wicked Whirl and Wind Shear) and debuffing you, then you might keep close to your buff and HP, use MED/MED/MED and SEN/SYN/SYN when necessarily. Don't let your buff gone to 0, at least Shell, Protect and Haste has to be ON.
Then all we have to do is race to death.

Added on June 15, 2010, 10:00 amI think is better hunt for the 6 bomb, more CP and easy to stagger and kill.
Don't worry about CP at the moment, you will have a lot of high CP mob to kill later.
Once you enter a battle, you can pause and click on RETRY, will not quit to the menu, unless you are not in a battle.
mission 64 the trick is just casting slow (is the main success).
if it dont get the slow curse....it will diff to kill.
once i found out this, the poison is not that important.
no need sen,syn,syn..normally when Imp. Aura u have enough time to cure & buff yourself (just haste, bravery,faith enough already).
then after that quickly debuff (fang only focus on slow, as light/vanielle dont have this ability, let them do the deprotect,deshell,imperil).
then start rav,rav,rav, once stagger...all com,com,com.
for me 3x stagger i beaten the boss.