Pretty easy to do. You can try to do it through story or since it is only 25 kills you can do Kill House on Realistic with low enemy density on Terrorist Hunt. When you start off at the train car, go to the left side of the building. There should be one guy on the platform between the two building. Kill him with a silencer on your weapon. Then climb the rope on the side of the building half way up. Fire a shot with no silencer. 2-4 guys should pop around the corner of the building on the middle of the map side, and then one guy on the other side. You will probably get killed after 4-6 kills, so just rinse and repeat and you should get this achievement in less than 10 min, depending on how quickly you move. The map reloads quickly on replays, so you figure 4 kills per try , you should only have to play the map 6-7 times, at about 1:00-1:30 per try. I posted a video I found of someone doing the story mode way so see which one works better for you!