Mission 13: End of the Line
At the start of the mission, you will have to silently kill some enemies:
1. This is automatically done through a takedown
2. Go up behind the guard near the traincar and knife him
3. Gather near the truck and kill the enemy to the left
4. After the garage door open, quickly kill the enemy on the right
5. After you gain access past some doors, follow the teammate to the right and shoot the enemy in the office through the glass.
After killing all 5 of these enemies silently, you will unlock the achievement.