Final Fantasy XVI isn't the first main entry in the series to do away with turn-based combat, but it's the first game in the series whose combat system comes with a surprising amount of depth, offering players a variety of different offensive and defensive options.
Among these options is the ability to parry enemy attacks, which has now kind of become a standard for action games. Unlike other action titles, however, parrying in Final Fantasy XVI can be rather challenging, requiring good knowledge of enemy attack patterns and timings.
Doing this consistently is kind of difficult at the start of the game, especially as the timing is strict. As your attack has to connect with that of the enemy, you must keep your cool and swing your sword at the right time, so use audio and visual cues to help you. Generally, pressing the attack button as the enemy readies its attacks is not a good way to go, as most enemies, especially the bigger ones, slightly delay their attacks. As the timing is so strict, and you risk getting damaged if you mistime your swing, it's always better to attempt parrying attacks this way only with bigger enemies, as their attacks are easier to read. It is doable with smaller, faster enemies, but it's ultimately not really worth it.
While not as advantageous, there are a few Eikon abilities that can be used to parry and counter long and short-range attacks, such as Phoenix's Heatwave, Garuda's Rook's Gambit, and Titan's Titanic Block. These abilities allow for easier parrying thanks to a much bigger window of opportunity, but they do not slow time as regular parrying does, so take all your options into consideration while defending against the most powerful enemies in Final Fantasy XVI to optimize both your offense and defense.