Unlike the other Story Mode achievements, you can feel free to alter the difficulty and game settings if you can't earn it with your own skills. If you're having trouble unlocking it, alter the Battle Setting to:
CPU-LEVEL = 1
DEFENSE LEVEL 1P = 8
DEFENSE LEVEL 2P = 1
POWER GAUGE 1P = MAX
Just make sure you're the first player so that you can take advantage of the defense boost!