This is a fairly easy achievement that you will unlock during your play through.
When you are in battle and the enemy attacks you, press the A button when the circle appears around the character that is being attacked. When done correctly, more specifically when done at the right time because this is all about timing, you will do a perfect block.
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!