To gain this achievement you have to deflect 20 attacks by the opponent using a "Guard Impact". This is different to guarding as you will momentarily stun the opponent afterwards and be able to attack them. This must be done outside Training Mode so the best method is to use two controllers.
Go into a Player vs Player match with two controllers set to 5 rounds and Infinite time and pick any two characters. At the start of the match practice the Guard Impact using one hand without doing anything with the second pad until you are comfortable with doing it. Best way is thumb on the stick and any finger on
.Your character will lunge forward with their guard and it will say "Guard Impact" under your character. Then try that while using your opponents Horizontal (
) attack at the same time as you do the Guard Impact and you will stun the opponent.
Stun the opponent 20 times to get the achievement.
If I can get a video demonstrating this I will do, I wasn't recording it as I did the achievement