I got this in my first try, at the same time as the "Get your Right Foot Wet" achievement, just by running ahead and knifing as many people as possible. Also try to keep a kill streak up and you can quickly go from 70-100pts a kill up to over 1k per.
I was on Recruit in "Hard Landing" and was the first player into the wall section at the end, I was able to knife everyone up to the first Flak gun and then mowed down a few dozen guys with the heavy artillery for upwards of 500 a piece. Also, I got into the Flak gun when you have to hold the wall and taking out the tanks was over 1k a piece there as well. In the end I had 22k and the next closest guy had 12k. Hopefully you get lucky with all 4 guys playing through the whole level as well.