You'll unlock the slow time ability quite early on during mission one in Coruscant, when you miss a jump and fall into a group of Stormtroopers. Push

+

to enable the slow time ability and kill as many enemies as you can during each use. To recharge the ability you'll need to defeat enemies. On story mode slow time recharges after every 3 kills.
Under the Force (Jedi Concentration) skill tree you can unlock Greater Hold and Extended Hold to increase the length of time each slow lasts for. Allowing you to kill more enemies within each use.
Defeat a total of 50 enemies over the course of the game while under the slow time effect and the achievement will pop. I found that you can also farm the same group over and over again, if you reset/meditate nearby.