I know this is extremely easy to get, but if you want to save some time, (small amount of time) you might as well not worry about this achievement. If you plan on going for the other online achievements, just join the BEANS BULLETS BANDAGES boosting sessions. Also to pull your knife out hit Y, if you immediately hit RT it double knifes. I couldn't stand doing this one(I did this by just running around and knifing) because you will knife people, and they won't die because the mechanics aren't so great in this game. Also the connection has to do with how your knife will hit them, on your screen you will knife them, but on theirs you don't. Sad I know..
Here is my link for full details on BFBC boosting sessions: