For this achievement you will have to knockout all the prisoners at least once in a prison.
The earliest prison to this is Rattlesnake Springs because there are only 8 other prisoners.
Edit 23/8/2017 (Credit: Nuttywray): Once you have unlocked Area17 there are only 4 other prisoners, so this is the best option to get this achievement quickly.
Tips: Once you get a quest to find a weapon for someone you can use that weapon to knock your enemies down easier.
Once you have knocked out your last prisoner, the achievement will pop.