First off, earn enough skill points to purchase EMPOWERED SLOW from the FORCE section of the skill tree. If you have already purchased this skill and have the next one along (BURST SLOW), that's absolutely fine.
Next, in the Settings menu, put your game on to 'Story Mode' difficulty (the shame!!
). To be fair, if you are on 'Jedi Knight' difficulty this should still work, as you should still be able to make a one hit kill on an enemy.
Next, find a meditation point next to a group of, or even just one, low level Storm Trooper(s) (that have blasters, not electric batons, although this method can still work on them too, but it wasn't as consistent as blaster storm troopers for me), so that you can rinse and repeat this method as many times as you need to.
Next, approach the Storm Trooper (you don't have to lock on to them, but can if you want) and when you get close to them (and ideally not being shot/hit by them), hold down the RB button. Cal will hold out his left arm towards the Storm Trooper and dependant on which skill you have unlocked, he will finish the 'slow move' by either lowering his left arm (Empowered Slow), or by spreading his left and right arm out away from his body to form a T shape, and then lowering them (Burst Slow). There will also be an audio cue to let you know you have done the 'move', and you should now see the Storm Trooper under the effect of Empowered / Burst Slow.
Now, your goal from here is to kill the Storm Trooper in ONE HIT (the 'one hit' part is very important). On 'Story Mode / Jedi Knight' difficulty, you should just be able to one hit kill an empowered slowed Storm Trooper by simply hitting X when close to them, and Cal will most likely stab his lightsaber straight though the Storm Trooper's torso, killing them in one hit (from full health). Cal may occasionally kill them with one slash rather than a stab, and I still think this counts?! All I know is that I did the above method 25 times (18 technically, as i must have got 7 trying the other solutions) on a single Storm Trooper, stabbing them through the torso for a one hit kill, and I got the achievement.
I'm pretty sure the key to success here, is the 'one hit' kill part, as you will often find that after empower slowing an enemy, after the first hit from Cal, the enemy goes back to normal speed, and I think this is tripping people up on this achievement (as the achievement says to defeat whilst they are still under the effects of empowered slow)! This method may also work on none Storm Trooper enemies??, but the key will be that you are one hit killing them after you have done the empowered slow, and not killing them in 2 or more hits, as the effect wears off after the first hit.