Before doing anything, walk over to the north side of the deck and pick up the book off the floor. This is the Al Bhed Primer vol. I. One down, 25 to go! Next, talk with one of the Al Bhed men to obtain a few Potions.
Speak with the girl at the south end to learn about using the Sphere Grid. Afterwords, she’ll tell you what your next mission is. Follow the chain underwater until you reach the next area. You may run into some random fiends along the way. Try to steal some grenades from the Piranha when commanding the girl.
Another Save Sphere is located at the entrance of these underwater ruins. Save your game or restore your HP if needed. When ready, examine the object nearby to open the door.
Follow the path until you reach a dead end. Examine the large object on the ceiling to turn on the power. You’ll notice a monster swim by during the short scene. Heal up your team and prepare for a fight before heading back to the exit. If you haven’t yet, teach Tidus the Cheer ability on the Sphere Grid.
Boss Battle – Tros
Once the battle begins, steal a grenade from Tros while commanding the girl, then use the Cheer Special ability while commanding Tidus. Cheer will increase both characters’ attack and defense. Use Tidus’s normal attack and the girl’s grenades to damage Tros. After a couple of hits, Tros will swim away.
A new Trigger Command will become available. Choose to “Stand By” for now. This will restore 50 HP as you wait for Tros. He will then swim by and perform a strong attack. With Tros in range, continue your attack.
Tros will swim away again after two attacks. Use the Trigger Commands “Stand By” for the girl and “Pincer Attack” for Tidus. This will prevent Tross from swimming away. Now all you need to do is eliminate it. Keep using Grenades and regular attacks. Use the Overdrive attacks if they become available, and use Potions if your HP gets too low.
Follow the girl to leave the ruins and get back to the Al Bhed Ship. Another lengthy cut scene will play out. Afterwords, you’ll get to save your game at the Save Sphere. If you didn’t get the Al Bhed Primer before, get it now! Speak with Rikku (the newly discovered name of the girl) once more to activate yet another lengthy cut scene.