I hope my comment helps.
I tried numerous times on Level 6 with easy, normal and a few times on hard no luck.
Then I tried Level 3-Subway section with the Gewher no luck. Then I pressed on until you can pick up the Vampire Gewhar with red sight. I then sniped the I believe 4 or 5 VG guys to finish the area then I proceeded to leave and progressed through the story until they ask you to kill the snipers. TAKE YOUR TIME they do not move. Then I finished up all rooftop / high level rooms with the VG. I aimed for the light of their rifle looks like a big white circle when in tracking / red mode. And proceeded to kill them.
1. I did the bridge between check points without reloading the game or dieing, doing so may not offer enough guys to kill but it should.
2. I did reload as it only holds 6 rounds
3. I am almost certain I missed a shot or two.
4. I did this on Normal
5. I did this without cheats enabled
6. I wiped the cache before loading / inserting the game.
7. I did not update the game, in fact I unplugged my network cable to avoid logging in. I'll update when needed to play and finish MP achieves.