After losing 350 HP, Tros will retreat to opposite side of the chamber. The party can no longer attack (unless the player uses Rikku to toss grenades) but can initiate a Trigger Command called Stand By to recover 50 HP each. Tros will use its Nautilus Charge attack which deals heavy damage. After 350 HP more is lost, the fiend will again retreat. This time Tidus can select Pincer Attack from the Trigger Command menu and tell Rikku to swim in front of Tros, while Tidus swims behind it. It will no longer be able to use Nautilus Charge.