At the start of a battle against an Eidolon, a time limit is imposed on you via a Doom counter. As you demonstrate your power to the Eidolon, its Gestalt gauge will slowly fill. Attack the Eidolon until this gauge is full, and then press Square to claim victory. Attacks are not the only way to charge the gauge. Using the Libra technique may provide you with hints as to which additional abilities can be used. Some battles have you use defensive abilities, like healing or Guard to fill the gauge. Filling the gauge is unique to the Eidolon you’re facing at the time.