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: Method 2: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.
Some other nervous system implants that slow down time can be seen in the image below: