When you're doing the first Carnales hideout mission, before you start knock down the fence next to the building and make sure you have an RPG with you.
Start the mission and instead of going throough the building, run to where you knocked down the fence and blow up the car.
If the explosion doesn't kill the sergeant, you have to.
The mission ends after you kill him since he was your target.
This method beats the heck out of wasting ammo on the guys inside.