I thought the control point challenge would be faster but it's stupid. I just shot a guy in the body & he didn't get up. So I'm glad I don't have to fool with it.
It's easy with a mortar turret as you never get a headshot with that. I did it on Navy Plaza:
The control point thing took me a couple tries. Shooting everything moving in the legs with the pistol in a tank build worked eventually. Don't use automatic weapons with a lot of spread and/or recoil as they would increase the risk of an accidental headshot considerably.