On the Tijuana mission where you get asniper rifle and the man says, "Now the Federales are after me!", you fight a group of men with rifles. They will be on the rooftop directly across the courtyard from where you are standing. To easily take them out, step to back into the corner and look just above the shack with the red roof. You can just barely see the head of each sniper, but they cannot see you. Take them out one by one until there are none remaining. You will save plenty of ammo by doing this. After that, go through the courtyard and around the corner. You are now rushed by multiple enemies. Duck behind the rear of the police car and snipe the men in the head as they come around the corner. You will save even more ammo by doing this. Around the next corner there are several more lambs to the slaughter. Duck behind cover before you go any further. Make some noise so that they will come around the corner exposing themselves. Snipe their heads. Note: One of them has a rocket launcher; take him out first if you get rushed.