Guidelines (MUST READ):
~Every enemy on the map needs to be killed.
~Accident kills are CONFIRMED (enemies killed by each other) will allow you to get this achievement. Just make sure everyone is dead. (Thanks to Ashyboi)
~Any difficulty is fine. Highly recommended to start new game on normal.
~You can repeat a mission if you fail to kill all the listed enemies. If you miss any, just press "play again" for the fresh start. (BE CAREFUL, DO NOT continue if you didn't kill all them. Level selection may not count.)
~Saves are completely good.
~IMPORTANT: On the right-hand side below the combat grading scale (rectangular box), "Total enemies" is the AMOUNT of ENEMIES LEFT on the level. You MUST match this with your "Events" Tab for "enemies killed". LET ME KNOW IF I NEED TO CHANGE THE NUMBERS LISTED BELOW. I have not played this game for quite some time now.

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Tips to go by:
In the beginning of every mission, start firing random shots to lure the guards. The music is a indicator for enemies around your position. When the music is completely gone, you can move forward to the next section until level is over. You should always circle the area and keep shooting your pistol. This is for your own good, the game might trick you so always be cautious or you'll have to restart.


Here is the official list of enemies in every level in the game:

~You MUST Kill Baker and the 3 guys by the door on "Nondescript Hideout" at the start of the mission.

Prologue City - None
Pro. Downtown - None
Pro. Steve's House - None
Druggie Ghetto - 15
Steve's House - None
Hideout - None
Druggie Slums - 38
Apartment Complex - 52
Abandoned Warehouse - 57
Re-purposed Factory - 44
Shopkeeper's Mansion - 33
Abandoned Complex - 22
'Luxus' Incorporated Complex - 41
Baker's Panic Room - 15
Steve's House - 8
Baker's Hideout - None
Nondescript Hideout - None
Empathestic's Rest - Part 1 - 29
Empathestic's Rest - Part 2 - 10
Drug Mob Casino - 35
Epilogue - 49
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And....That's about it. It shouldn't be too hard. Just keep using your saves and you'll be fine. Cheers!

Updated Note Since 12-17-24:
-You CANNOT skip the kill scenes during the missions by pressing ESC. It will make those kills not COUNT for some odd reason. - Thx to @Turncloak