Sinspawn Ammes is the first boss in Final Fantasy X. It is fought by Tidus and Auron at Zanarkand after Sin's initial attack. The battle is used as more of a training exercise for the player, as it includes instructions on Overdrives.
There is little chance of defeat, as Ammes' only attack is Demi, which cannot kill you since it only takes off a percentage of your health, and at worst Auron and Tidus' HP falls to 3. There is still the possibility of being defeated, however, as the player can still attack themselves.