You can actually do this with only 2 enemies... maybe 3 to allow for mistakes. If you get to a point with 2 or 3 of the swordsman (not the blue security ones in the first few missions). Play on ultraviolent with completely un-upgraded sais, I was able to get to 300. You need to just play them off each other and keep an eye on the B highlight to combo them so they don't hurt you...
The camera is the biggest hindrance at this point. best is to find a wide open area where you can see them coming, or a tunnel where they are always behind or infront of you.