For this achievement you must have a cyberware implant that slows down time equipped so that you can kill an enemy while time is slowed.
Method 1:The cheapest cyberware implant that slows down time is a nervous system implant called
“Kerenzikov” which costs $5000 at a Ripperdoc (scissors icon on the map).
* Kerenzikov allows you to aim and shoot while dodging. Slows time by 50 percent for 1.5 sec. when blocking, aiming or attacking during a slide or dodge. Cooldown 5 sec.
I used the Ripperdoc in Japantown to purchase this implant.
Method:Weaken enemies by shooting them until their health is low. While sprinting press
to slide then quickly hold
to aim and slow down time for a short time. You must kill the enemy in this short time window. Shotguns, revolvers and pistols work best. Kill 50 enemies like this and the achievement will unlock.
This is a video of how it is done:You can purchase the
“Dynalar Sandevistan MK. 1” operating system
which costs $7500 from the western most Ripperdoc in Northside, Watson.
To equip this item, you need 6 Reflexes.* Dynalar Sandevistan MK. 1 slows time by 50% for 8 sec. (Cooldown 30 sec.). Increases any damage dealt by 5 percent when Sandevistan is active. Activated by holding 
. Once you have it equipped, find a group of enemies (story missions, side missions, around the open world or police), activate it to slow down time and kill enemies while time is slowed.