It really doesn't take that long, you just have to know how to snipe and how to set up your loadout. There are tons of people who have ghillie now so it really isn't as difficult as you think. Plus, get the variable zoom done at the same time and then switch to thermal while wearing your half-ghillie if you want.
Killing 850 opponents is not a challenge.
That's a "honey do"-list.
A challenge would be: Kill five opponents with one shot with a sniper rifle.
Or two opponents. Five times. In one game.
Or, seriously this time, "get 7 headshots with sniper rifles in one game", in a total of 10 times.
A challenge should require some amount of skill.
Be happy its 850 instead of 970 ^_^
Face it: tier 3 challenges are meant for long-terms only, not to be done within the two weeks, else you need to use only one loadout for the whole period.
If you have issues unlocking it, lock it so it won't go away and take your time, no one is pressuring ya, and it will get eventually done.