Things You'll Need
Instructions
Beat the game. This will give you access to the 10th level of the crystarium, which you will need to bring Hope, Fang, and Snow's HP to over 10,000 points each.
Return to Gran Pulse and activate mission 54. You will find the stone for this mission on the Northern Highplain of the Archylte Steppe.
Head to the star on your map after activating the stone. Make sure you have the following two paradigms: Hope (Synergist), Fang (Sentinel) and Snow (Commando), as well as Hope (Medic), Fang (Sentinel), and Snow (Commando). Optimize all of their equipment for defense, and make Hope your party leader.
Use your Fortisol and Aegisol before you begin the battle.
Start the battle with your Synergist/Sentinel/Commando paradigm. Cast the spell "Enfire" on your entire party, then switch to the Medic/Sentinel/Commando paradigm.
Have Hope heal Snow, Fang, and himself as needed. The Gigantuar has one attack (10,000 needles) that damages a member of your party for 10,000 HP. While you are busy healing from these attacks, Snow will attack Gigantuar with physical attacks augmented by fire (Gigantuar's weakness). With only 1,111,111 HP, the Gigantuar cannot survive such an onslaught for long.