At the Deep Research Facility, the second hardest optional boss in the game and the second Weapon is available to battle there. The Ultima Weapon is probably the hardest battle to reach and fight to boot. First the battle will rely on your party's limit breaks for basic damage and for your defense and resistance increasing abilities.

Make sure you draw some critically important magic from Ultima Weapon, Regen, Dispel, Ultima and if you missed it, the Guardian Force Eden.