What you must do for this achievement is get 100 preemptive strikes consecutively ( in a row ). I did this early on in the game in the Bresha Ruins, in the underground tunnels, being as it seemed most of the enemies are clueless down there.
As for some tips:
1. Play on easy.
2. As soon as the Moogle clock comes up, strike fast! If the clock runs out or the enemy hits you first, you will have to start all over again.