Instructions
Change your paradigm to Sentinel at the start of the fight. This paradigm allows Snow to use defensive moves.
Select "Abilities" from the combat menu, then select "Steelguard" enough times until your ATB sequence fills to its maximum.
Wait for Nix to strike. Just before she moves to launch her attacks, submit your Steelguard abilities to begin blocking her attacks.
Continue with Steps 2 and 3 until the Gestalt bar fills up, then press "Square" (PlayStation 3) or the "X" button (Xbox 360) to complete the first Eidolon fight.