This is obtainable in the Bangkok level, after you complete the vehicle chase scene with the trucks. This is cumulative, so you should be able to get this with no trouble.
After the the cutscene with the cops looking for you, you will start crouched behind some crates. As two cops walk away, wait for the first cop to walk over towards you, and preform a stealth takeout to get his stun gun.
Follow the others around the corner and stun the first cop leaning against the car. Grab his ammo, and take out the guy on the left, Run over and grab his ammo, and repeat this with the third and final guy in this area.
From here you can either reload the checkpoint, or proceed to the next area. Hide behind the white/red block, and wait for them to separate. You can then take them out without being scene.
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