In order for you to obtain this trophy, you must beat the 18th Mission, Cobra Manor Takedown, having reached no more than 19 civilian casualties.
If you have surpassed this amount when you beat the final mission, don't panic! Despite seeming like it would be missable, it actually isn't because one of the collectables you pick up allows you to reduce this number by half for 66,666 credits a time.
Providing you haven't gone too mad with your spending and have been opening crates pretty consistently (esp. the purple hacking ones), you may not even need to grind for it. Just by going through Nightmare difficulty, I had all the credits I needed to reset it 3 times and bring it down to what I needed.
The collectable you need to find:
*** Spoiler - click to reveal ***Sin Eraser: This is found in level 12, Circuit Market, it's in plain sight but requires the Old Key to access it.
To get that, go to level 13: Industrial Boom with Shrinkify and Hacking Tool equipped, defuse the bombs, collect the Blue Key from the purple chest in the bottom left, then go to the top and between the left wall where the collectable is and some pipes, you can squeeze through. That part of the map will then light up, revealing a blue door and you can then use that key to get the collectable.
Once you have reduced it to 19 or fewer, beat the 18th mission again and make sure none of the civilians die, particularly if you're on the threshold or within a couple (use the Citizen Health Plan perk to hopefully avoid that scenario!)
Best of luck!