I did it in the 1st mission. You can get it right at the start of the mission, and if you fail, you can quickly restart.
Start shooting with your rifleman while parachuting. Take down the guys running towards you by shooting at the barrels. You can even blow up the vehicle with this method.
After landing, you should already be around 5-6X, so simply (but quickly) take down all remaining enemies without losing the multiplier. I ended up with 9X a few times, but managed to get it after a few tries.