To be able to deflect an opponent's physical attack you have to press LT (block) just before the enemy attack you. If done correctly the attack will deflect off your shield and the enemy will stumble back a bit.
If you for some reason can't get the achievement against the CPU you can use a second controller and go to Duel to get this very easy.
Do this 5 times in a single battle to unlock the achievement.
*Credit goes to Sakura460 from x360a*