The Eidolon knows the truth. Ha! I knew I was right.
Hecatoncheir is a boss in Final Fantasy XIII. He is Vanille's Eidolon, and is fought by Vanille and Fang in the Mah'habara Subterra's Flower-filled Fissure, as a test to prove Vanille's strength to him.
Like all Eidolons, Hecatoncheir inflicts Doom on the party leader at the beginning of the battle, and instead of being defeated, Vanille must fully fill up his Gestalt Bar. Hecatoncheir goes through stages of charging and attacking.
Hecatoncheir is fought with only two party members, making the battle a bit tricky.
By pausing the game and selecting "Retry", the player can set the paradigm deck before battle. Recommended Paradigms include Divide & Conquer (SAB/COM), Stumbling Block (SAB/SEN), Lifeguard (MED/SEN), and Slash and Burn (RAV/COM). Hecatoncheir is immune to any of Fang's Saboteur abilities, so Fang should be a Sentinel for most of the battle.
When Hecatoncheir is charging, the party can use offensive paradigms, but Fang must switch back to Sentinel when the Eidolon starts attacking, lest Vanille be attacked or KO'ed. If Fang has learned a few Ravager abilities, Dualcasting (RAV/RAV) can be included to quickly increase the Gestalt Bar. However, the player should make sure to have used Fang as a Commando to stop the chain gauge from depleting too fast, and switch to Dualcasting to build up Hecatoncheir's stagger meter. It is a good idea to attack relentlessly if the Eidolon is charging using Looming Wrath.