You need to avoid detection starting at the point where you cross paths with Chris and Piers, which will be Ada Chaper 4, Part 2.
There are a few enemies that have to be killed, so use regular arrows, no guns.
*If at any point you're spotted by an enemy, quickly hit and restart checkpoint to continue from that section without penalty. Also note that dying and hitting "yes" to continue will NOT allow you to earn the achievement. (Credit to this piece from another player on xbox achievements)
One you reach the area where Ada part 2 is dead on the ground a cut scene will start and you should get the achievement to pop shortly after.