Oh, be nice!

Here's how my characters were if it's of any help:

Ultima, Life, and Full-Life on everyone's elemental defense, making them absorb all elements. Strength stat near the maximum (strong magic for this can be found at the Island Closest to Heaven/Hell) and lots and lots of Aura.

In battle, I summon Cerberus, triple Regen and Aura...sometimes spells like Haste, Protect, and Shell, but mostly just Regen and Aura, then go insane with the limits.

If you have Bahamut's card, it refines into 100 Mega-elixirs, which is pretty handy in Ulti's last form.