Performing an impact is simply put as a "Guard Impact". To do a guard impact hold back in the directing you aren't facing and press A+B+K just a little before your opponent lands an attack, if done correctly you will deflect their attack and this green burst will show. Do this 100 times and you'll earn the achievement. This can be earned in all modes except vs and practice. You can however boost with friends.
Tip: Some characters such as Xiba, Maxi, have auto GI's that don't require critical gauge. Others such as Siegfried/Nightmare (during their critical edge)