Allow your partner to snipe 5 enemies while in Mock Surrender
Go into mock surrender by tapping when given the prompt, and just let you partner snipe as many guys as possible. He may need to attempt this a few times to get 5 kills.
Easy method for solo: load up chapter 4 (start) and have Rios/Salem advance () to the corner and have him stop (). Now sprint up; you'll see people running. There is a yellow washer machine here (black cable next to it if you didn't see it right away); duck in to cover here. Stay put and let the guard take a step forward before coming out of cover (if you don't do this, they'll just fire on you). Ease out and go into Mock Surrender. Rios/Salem will target the guy on the ground you shoot the guy in the turret. Restart checkpoint.