Mission 14: Sin City
After going through the kitchen area, you'll want to kill all the enemies near the two staircase. After going down to the lowest floor in the area, you'll find 20 slot machines (4 banks of 5) to the right hand side. Make sure to use you gun and shoot the screen on all 20 of them.
Next, progress through, and let your teammate open the shutter door for you. Once you crouch under, go down the hall and BEFORE going down the stairs, aim across the room to the balcony in front of you where you will find more slot machines. Shoot 1 to unlock the achievement.