Just found a good place to unlock this!
First level, make sure your Sniper is equipped.
When you parachute in, drop into the Sniper Tower, you'll know which one this is, it's by far the tallest building in the level, very thin, just to the left of the 'Main House'.
Land in here, there will be an enemy but he won't see you coming from above - kill this guy off anyway you want.
Now make your way down the stairs in the tower, having a few pot shots from the gaps in the windows, it is not essential you get any headshots here, but you're just clearing a path.
When you reach the bottom of the tower, look right, jump over the sandbags, then follow the road to the right.
Now, you'll notice the road will slope downwards and to the left, this is where you want to be.
Your team will be here supporting you, and will respawn, as will the enemies, as long as you stay at the top of this road, and don't head down to the archway.
After finding this I picked 10 off with headshots straight away, and I found the shooting in the game pretty difficult.
Just take your time, there's a very slim chance they will kill you, line up your shots, make sure the AI is stationary, then BOOYAH!