The way I got this seems to be different than others, but bear with me.
I got this achievement after downloading the map pack for the game, and realizing how awesome it was that they brought back Calypso Casino from RSV 1. Put the wave density for however much you want, clear the terrorists at the start (I'd suggest a long range weapon) and proceed down the stairs and stick to the left side of the map (from the perspective of the stairs). At the end of the roof, there are two rappel points, and when you take out the on the one on the lower floor while inverted on your trusty rappel, 4 or 5 others will cluster around him. I got around 6 guys at once with that, then you invert again and look to the top of your rappel point, there will be 1 or 2 other terrorists standing there. Just peg em, and you have 6 or 8 already (Don't miss or they freak out and start shooting at you, and you have a pistol, they usually seem to have LMG's).
When I got the map pack, it was free. Not sure about now, but regardless, I hope this helps.