Finally figured out how to get Gold medal on Challenges.

[Rooster6975]

You don't actually need to combo nearly as much as I originally thought. Just stealth kill as much as you can at the beginning of every level until they set off the alarm. Use the knife, aim for the head. In the prison challenge for instance, a commander is walking away from you when you start. If you throw the knife at his head, you kill him for 1500 points (900 for killing a commander + 200 bonus for stealth + 200 bonus for knife + 200 bonus for headshot). Then you can easily headknife the second guard for another 1000, and have time to headshot the third guard on the ramp with the pistol for 800. That starts off the round with 3300. Then you can get the dog with the knife as well which should give you another 900.

That is more than enough to make up the shortfall. I kept finishing less than 1000 short of the goal, but with no one left to kill. Use the knife as much as you can because even if you hit a soldier in the foot, you still get 600.

[Nobody Cares]

Well for prison level it is good to start with stealth kills but maily it is necessary to keep one officer alive (valid for all levels with officers) so they can call reinforcements. Also usually it's enough to kill with knife ~2 enemies at start and then just chain headshots as much as possible, going for explosive kills etc. Heavy soldier is very nice walking bomb, aim for their backpack, easy to shoot it also from front :).
Problems going to start when zombies appear (last 3 levels I think) as they will steal your kills, so take care of them as soon as possible.
And one last thing, you can check this video, there are some hints how to beat all challenge levels. Worst one for me was Workshop level, rest are easily doable at 1st or 2nd try.
https://www.youtube.com/watch?v=MR8R81a_nxQ