Tidus' strength could only be determined by Squall's level anyways, just like anyone else Squall fights. So if Squall were at a low level like at the beginning of the game, it would be roughly the eqivilent of Tidus with an empty sphere grid. In which case Squall would lay a royal beating upon him. But if Squall was at top level, Tidus would have a full sphere grid and be able to top the gunblader. Think about it...