Sniper Elite: Resistance

Any kill executed from tall grass while crouching (Hawker will have a white outline around him when you're hidden), be it with your rifle or a silenced pistol will count towards this, as will sneaky melee kills. Most of our kills came from close-range stealthy silenced pistol shots with subsonic ammo, and 50 is a small number. You can do this during a single playthrough, no sweat.