This can be done in the second mission, Bitanti Village, against the first two enemies you come across. Make sure to take the welrod pistol, as it is the weakest pistol in the game. Simply take out one enemy, while the other is not looking. Then run up to the second enemy and shot him in the foot, this should cause him to fall down and become incapacitated. Now switch to your sniper, aim between his legs and fire.
Here is a video guide with English commentary!
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https://www.youtube.com/playlist?list=PL-HeW6XkKdUJ_Be1KEgJ2...