This is a fairly easy achievement that you will unlock during your play through.
When you are in battle and you choose to attack an opponent with your weapon, hit either the A, X or Y button when your weapon starts to flash and you will do a perfect attack. (which does the highest damage to the opponent.)
Do this 100 times and the achievement will unlock for you.
If you find the solution not helpful, please let me know why so I can edit it!