Sin will charge directly towards the player on their airship, moving slowly forward turn by turn while slowly opening its mouth. After about sixteen turns, Sin's mouth will open completely and it will use its Overdrive, Giga-Graviton, to destroy the player's party, resulting in an instant Game Over.
For the first half the battle, Sin is out of range of physical attacks. Its only attacks, outside of the massive main attack, are status effect moves. It is not possible to protect against Giga-Graviton, as it bypasses damage altogether and simply causes a scripted Game Over. Even summoning an aeon will not prevent this.
It is possible to end the battle quickly by having Rikkumix a Trio of 9999, wait until Sin is in range, and use multi-hit attacks on Sin, such as Tidus's, Lulu's or Wakka's Overdrives. A strong Anima's Overdrive can KO Sin in one hit.