Colt Canyon

The most difficult part of this achievement is that you must do it in one life, no saving/reloading if you get caught. I tried killing 6 or so then saving so I could go back to that state if I got caught, but if never unlocked for me. You will know if the kill was a stealth kill if the incognito icon in the lower left corner of the screen is active after a melee kill . I never threw any melee weapons and unsure if this affects the achievement. If you get caught you can still get this in the same game but the count resets.

Settings I used:
Seed: 060A7690F - Has camouflage perk with first prisoner, you can pick this up first to make it easier.
Character: I chose "Mom" since she avoids detection and with her Pan starting weapon (you may have to use her a bit to unlock). It can kill all 15 without it being destroyed.

Using the same seed you can eventually learn the position of the enemies. There are more than 15 in level 1-1 so it is suggested that if you get caught, just choose restart from the pause menu. I don't think civilians or dogs count, but dogs can alert enemies so best to get rid of them. Even if dogs spot you it will not remove your incognito state. Always melee kill enemies when they are alone and when they do not have the question mark above them. If they do, wait for it to cool down.