Mission 6: Hunting Party
As you reach the hospital, take out some enemies, and then bust through the blue door, some people will be running out and some enemies will be behind them. Take out these enemies, Run up the hall, and through the first door on your left to spot an enemy in a down but not out state sitting on a gurney. Take a smoke grenade (you start the level with them), and hit him directly with it to unlock this.
Possible on any difficulty, but recruit is probably recommended to make it as easy as possible.