Instructions
Attack Nix twice right off the start of the fight. That starts the Gestalt gauge moving.
Shift Snow into Sentinel mode. Ravager and Commando aren't needed once the first attacks have been completed.
Use the "Provoke" ability. This gets Nix to attack you.
Cast "Steelguard." This is the most effective way to force the Gestalt gauge to continue rising.
Watch Nix's Gestalt gauge carefully throughout the battle. Press the action button as soon as the gauge is full. This is how this battle actually ends, so it is very important to keep an eye on the gauge throughout. Nix casts "Doom" at the start of the battle, and if you don't press the button with a full Gestalt gauge before the doom timer finishes counting down, you lose the battle.