This is an ability you learn early on in the game, so you should unlock it over time.
1) When fighting enemies with lightsabers, batons etc. watch for them telegraphing their swings, when they do, hit the LB as they swing to achieve a parry.
2) If you achieve a parry you should notice them become stunned or stumble.
Repeat this process 100x and the achievement is yours. Good luck!