First of all, while there is a fighting tutorial required before being able to play the game, please note that there is an advanced tutorial that teaches really important techniques, including parrying.
This tutorial can be found by going to the How to Play menu.
Parrying is done by blocking in the correct direction of an opponents attack, then pressing the heavy attack button (RT by default) at the moment that the attack icon flashes red.
Doing this 50 times will unlock this achievement.
What I noticed was that when looking at the achievement tracking for this achievement, there was no way that I had gotten as many as the achievement showed while in PVP matches. It looks like any parries will count for this achievement.
Specifically, if you want to grind this out quickly, keep practicing parries in the advanced tutorial until the achievement unlocks. This is what I did.