Tidus washes ashore and meets Wakka.
BOSS: Kimahri
HP: 750
Weakness: None
This is a tutorial duel. Just attack. If your HP gets low, use a Potion. He will yield after taking enough damage.
Later, inside the temple:
BOSS: Valefor (Aeon Battle)
HP: N/A
Weakness: N/A
Yuna must defeat Valefor to earn its allegiance. Use Blizzard if you have it, otherwise standard attacks work.