The best way to start "Culta's Prisoner" mission is to be on the mountain's side. From there you can see the whole battlefield with all enemies. Use binoculars or drone to tag enemies for synchronized shots. If you have a sniper rifle with a silencer, you can eliminate the enemies. But if you only have an automatic weapon, focus only on tagging the enemies for your allies.
The gap between synchronized shots must be thirty seconds. Remember to tag enemies in a certain way so those that are alive won't discover their dead comrades. When this happens, they can call for helicopter support. It is hard to hide from it or run away.
When most of the area is clear, go with your team next to the church. There may be one enemy inside of the building.
Open the cell and speak with the prisoner. This completes the mission.