meh not really, i got my pistol one and the 400 kills one at one kill away each and then just went gruntpocalypse for last kill. Get 2900 cR each time.
all you really need to do is combine challenges so you get more like today you can combine 3 of them
1 400 kills in firefight
2 40 kills or some thing with pistol
3 100 kills on outpost
just get 39 kills with pistol and 99 kills on outpost then go to a different map and get 399 then save and quit and then go back to outpost for that last kill with the pistol you will get 3700cr per unplug which is a hella lot of points