Here is a 100% working solution. Achievement is not glitched. Already had the achievement on Xbox One, so i did this on PS4 version.
Load Mission 8.
- Kill the 2 soldiers down the path with Welrod.
- At bottom, kill the lonely soldier with Welrod.
- Do not alert or kill the 2 near the table on left room.
- Walk over the green narrow bridge. Kill both soldiers in that big room.
- Walk up to sniper nest, but do NOT activate it.
- Save game.
- Activate Sniper nest.
- There is 1-2 soldiers near the fireplace to the right. This will alert enemies near the table
- Wait for them to walk to left, then to right in front of your sight. Kill both.
- DO NOT SAVE GAME
- Reload last save
Achievement tracker will pop up after every kill.
You are guaranteed 3-4 kills each time.
You can move a little bit in sniper nest, and the kills will still count.