You should unlock the slow skill early on in the game on the Force side (left) of your skill tree.
To use empowered slow hold the RB while looking at an enemy. Once they are slowed make sure their health is low enough to kill them or hack away until they are dead. Do this 25x and the achievement is yours.
**As some have noted in the comments, make sure you HOLD LB for a good amount of time and you’ll see Cal’s posture/stance change and you’ll know it’s an empowered slow. After that, kill the enemy.**