An alternative to the above solution, you could simply use your thumb and index finger to press the button quickly.
This method has worked for me on all games that require press a button rapidly, not just on Blue Dragon. Therefore this can be applied to all achievements similar to this one. It does, however, take some practice to master, but overall it is effective and easy.
Position your index finger and thumb finger as if you were trying to pinch something; or pickup a hair off of a table. The key is to assure your fingernails are covered by each other as best as possible to avoid them getting caught on the edges of the button. (It's best to use your dominate hand as the pinching hand).
With your other hand, brace the controller tightly and maybe even up against your stomach or chest. This is important to maintain consistency between your presses.
Now, simply place your pinched fingers on the button desired (for this achievement, "A" button). Rapidly strafe your pinched fingers to the left and to the right of the button, approximently enough to almost touch the right joystick's protrusion (the part that begins to rise up from the flatness of the controller that leads to the right thumb stick) and as far right as almost hitting the edge of the "B" button. With practice, this motion can be easy and do the trick very effectively. The trick is to vibrate your center of gravity (based on your entire arm) at your wrist, letting your elbow sway freely. The energy thus goes into the fingers propelling a fast back-and-forth motion, and the energy also moves to your elbow, and transfers back into your wrist and then into your fingers. Try it out, and you will see what I mean!
Note: The back and forth swipes should be very fast and very brief. A good benchmark would be to aim for 6-8 presses per second.