There is a one-in-three chance that Oblitzerator will counter with a barrage of blitzballs. For physical attacks, the counter causes Darkness; for magic, they cause Silence. On its turn, Oblitzerator will use Blitzball Rush, which randomly hits the whole party for high damage.
With 6,000 HP, Oblitzerator is a difficult battle, especially since its Agility will increase with every turn and Yuna is not available to heal the party or call an aeon.
Tidus will notice a crane on the deck, which needs to be hit with Thunder three times before Tidus can use the Trigger Commands to activate it and remove Oblitzerator's top half, weakening and lowering his HP to 1/16th (a maximum of 375).