Heres how i do it, i have 3 kill streaks, harrier, airdrop and emergency airdrop, we do 9 head shots each and we end at 18 head shots each. i let you pick up anything u want, even emps, chopper gunners, and ac130s. if u get any choppers, i shoot them down. ill also help you get cold blooded pro, and cold blooded pro (V).
so in the end, you get a lot of exp per round, because you unlock many challenges, ones you weren't trying to beat.

but if you want to just do head shots that's fine, i just want exp.