Through the looking glass
Kill 10 snipers before they see you
The achievement / trophy seems to be glitched for many people. As far as i know there is no real solution for this problem right now.
Early in Mission 5 you will see a cut-scene. After it, go into the the small room right next to you. Thats the spot were you can get the 10 Sniper kills.
Look out the window and wait for the enemy sniper on one of the rooftops. I highlighted the 3 spots he will pop up in the video. He patrols this roof and moves between 4 different spots. One Spot is on the backside and he will be gone for about 60 seconds. Just wait for him to come back to the front side. As soon as he shows up make a manual save so you don’t have to wait for him after loading. repeat this 10 times and the achievement is yours.