To achieve 100% accuracy you cannot miss a shot during an entire mission. Probably the easiest mission to complete this in would be Act 1 Mission 3. There are a total of 41 enemies which can easily be killed with your sniper or stealth kills. Just take your time and aim carefully.
Another mission this can be obtained in no time would be Act 2 Mission 1. There are a total of 20 enemies. You will only need to shoot 11 of them. Later in this mission you will destroy two AA gunners and run into an underground tunnel. Run into the shed to hide from the patrol. As soon as you see the last enemy walk by, quickly run straight and then make a right and keep running ending the mission.
If you miss a shot, you can restart from last checkpoint.