Cumulative
I unlocked this during my first battle in the Tulira ruins, 27 hours into the game. Almost every attack has a perfect timing to get a critical. Do it 500 times for the achievement. I don't believe good hits count. I would recommend trying to learn the following, as I found them easiest and these members will be with you the most:
Cris bell's basic attack, when you hear the first hit
Christopher's spells, like when fireblast's shadow disappears
Zas's basic attack, when the hammer is high at the enemy's head