The sawed off shotgun is by far the best gun- I didn't even get below 95% after I started using it. Use the sniper for secondary for the pesky runners and its pretty easy. Too bad 7 people is the cap, I kept having extra points to spend at the end.
Edit: keebs beat me to it. 20 is easy too, you just need the sniper for your secondary to hit the army guys in the head.