In Mission 10: Ambush, there is a sneaking section early in the mission (after attacking the truck convoy). You are armed with a silenced pistol and must infiltrate an air base. Stay to the far right side of the base. Kill the enemies along the way with a headshot or 2-3 bullets to the upper body. There is one room with three enemies and the base commander -- just quickly shoot them. It is fine if single enemies see you as long as you quickly kill them afterwards. They just are not allowed to alert the entire base. If the icon above their head turns red, you still have a second to kill them. Restart from the checkpoint if you are detected. You will get the "Infiltrated" achievement after climbing the ladder to the sniper nest.