For an Action Camera moment you must kill around 4-5 enemies with a singe grenade or rocket. In mission 3 after you try saving the priest their will be an MG42 nest and a group of enemies. Label the MG42 nest with your Bazooka team by holding down and aiming at the machine gun nest. If done correctly the bazooka guy will shoot a rocket at the machine gun nest and once it hits it will trigger an Action Camera moment.
Strategy: Best tip I can give you is to use the bazooka team on any large number of enemies you encounter. Let the bazooka team open up fire upon the enemy and not the other way around because it's easier to hit enemies while they are standing still and not aware of your presence.