Complete one Terrorist Hunt on the highest difficulty, which is known as 'Realistic'.
You can, if you wish and to make things easier, put the density down to low, which means there'll be less enemies and in smaller groups, also switch lone wolf to 'No', this means you'll have 2 A.I. teammates with you.
The quickest and easiest map, in my opinion, is the 'Streets' - Select the 'G36C', 'M8', or the 'AK47' as your primary weapon, and fit the 'Rfile scope', also take some frags and C4 if you want.
At the start; go up the ramp to your right and press
on the left side of the upright box to the left of the ladders- position your teammates so they cover the ladders.
Now go the other way down the corridor, there are some stairs on your right, (you can plant some C4 just before these stairs if you wish), and go about halfway up and shoot your gun to attract the enemies outside. Now run back to the ramp and take cover on the wall to the left of your teammates and cover the corridor you just ran down.
Now simply wait, the enemies will come down the ladders and your teammates should take care of them, any one else will come down the corridor and you have them dead to rights. Keep half an eye on your right, as sometimes your teammates don't cover the ladders properly- you can, alternatively, place them behind the box to the left of the ramp to cover the ladders from there.
If you wait long enough you can get every enemy on the map to come to your position there and win. However, if you get to around 10-14 and no one is coming after a few minutes, move your teammates up the ladders.
Go up with them but be cautious, put them to the left behind the boxes then take cover on the right wall there just in front of the ladders. This will trigger the final enemies, keep an eye on the doorway to your left and finish off the map.