First of all, you should boost your created fighter parries skills (striking skills, last on the right)
Then, simply go to challenge mode with your created fighter, then, in the striking challenges, choose the 6th one to learn and test parries.
Hit rb as soon as your oppenent strikes (or mash the button) and complete the challenge.
Repeat five times ( changed since the update, thanks to ScuzzyBunny for the tip ) to earn the achievement.