In mission 10 there is a stealth section early in the mission. You are armed with a silenced pistol and must infiltrate an air base. Stick to the far right side of the base. It's okay if single enemies see you as long as you kill them quickly afterwards. They just aren't allowed to alert the entire base. You can restart checkpoints when detected.
Here is a video guide with English commentary!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUKe25WzwEqI...