This foe has two kinds of attacks it uses frequently: Tracking Missile, which is able to inflict heavy damage, and Drain Claw. Zack best having high Vitality to withstand G Fine's attacks. Graviga can be used to deplete its HP, continued with a sequence of Costly Punch. Drain Blade coupled with a high Strength stat helps too. It's best avoiding using magic other than Gravity and Graviga, as Zack's MP should be reserved for healing.