I did that commendation in Jefferson Plaza, right at the beginning just go down the stairs, pass the tourniquets, stand there and shoot on the balcony which is on the opposite side of metro hall. There will be a sniper and one more soldier (depends on difficulty - just switch between story and normal), shoot them, change the difficulty then rinse and repeat.
If you´re bad at sniping like me, just lower your mouse sensitivity a bit in game options, it helps a lot.
BTW this is also a good location to do Battlefield Awareness Distinction (From an elevation position, take out 3 hostiles with your primary weapon, then take out 3 hostiles with your secondary weapon, without missing a shot or taking damage), best with two sniper rifles equipped.
And Prone Target Award (Use any offensive skill to inflict damage on 10 prone enemies) as well, because there is one LMG guy at least guarding the area.

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