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.
The first 20 are in the section just before you go under the gate that one of the guys are holding open. They're all next to each other and like #2 said, the fat ones don't count as they can't break. The last one you'll need is after you go under the gate, there's a curved escalator. If you look straight forward from the the top of the escalator, there's a balcony with another set of 5 on it. Just shoot one and you're good.
Once you reach the casino in the mission sin city there will be 20 slot machines destroy them and the next 5 are in the next area near the escalators go ahead and check out the video and you should get it in no time