You unlock this ability in the prologue level, as soon as you have access to your lightsaber. It requires that you tap
the moment before a strike would land; on casual difficulty this timing window is incredibly forgiving.
In the very same prologue level, you end up on board a speeding grav-train, where the game first introduces you to the concept of blocking, parrying, and slowing an enemy with the Force. In this small segment, you're locked in a room with an infinite spawn of baton-soldiers, who drop down from the roof one at a time - as long as you don't complete the tutorial objective (either hold
to block or use
You can easily grind the achievement out in this room, grabbing 7 or 8 kills every minute.