The Sniper Team bonus mission requires a measured approach. In the first area, shoot the far tower's enemy, then the one on the left, followed by the one on the right. From there, proceed to take out the ground troops quickly (you can risk shooting an enemy beside his friend if you take the 2nd one out fast enough).
Head down and through the next building, waiting for the two guys to finish talking before taking them out. Turn off the light before heading outside. Head out and down to the lower level. You don't need to take out the three enemies to the left; just crouch-walk across the bridge to the right and climb the far ladder, heading straight into the building ahead. Heading through the building, you can bypass all of the enemies if you're patient. If you want to see the path I took where I killed several of them, check the video below.
In the next area, head down and assassinate the enemy by the fire pit, then wait for another to approach from the left facing out. Kill him when he gets close, then pick off the remaining enemy on the far side, leaving the two on the right alone. Shoot the enemies out of the sniper towers, then mop up the two on the right. Head to the end of this area and down a set of stairs to end the mission.
I've included a walkthrough video of 7 of the bonus missions below. The Sniper Team mission starts at
16:42