How to beat Hecatoncheir with just Fang and Vanille

Vanille on Hecatoncheir

In a nutshell, Level up!

Not to put too fine a point on it, that's the whole trick. You'll grind back and forth through the tunnel, the whole way back and the whole way forward,  again and again, over and over, until it's second nature and you rarely if ever get k.o.'d.  You have no choice about this — the game is linear.  Get past Hecatoncheir, or give up.

When you're done, Vanille and Fang will both have Level 4's in two or more abilities, plus maybe a Level 3 or so, with weapons upgraded to level 12 and 24. Status should show both characters with HP over 3400, not counting accessory bonuses.

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N.B. —  If you get lost trying to make sense of "Gestalt bar," "Stagger bar and "SP bar," the Mumbling Mystic Masters of Button Mashing Mayhem at IGN, etc., are trying to obfuscate and confuse with  technical short-hand wizards' vocabulary to impress the girls they imagine are also playing the game.  

Sad.  The only bar that counts is the thin narrow bar that gradually fills up purple (if you're spamming Synergist spells), with the illegible word under the bar, flush right.  When that fills to the word, hit Square.  

This topic is discussed professionally in the Datalog (green up Triangle), emphasis on clarity.  If you have an HDTV, the word under the bar is legible.