I have some ideas that might work out.
1. If a pokemon spots you while you're trying to take its photo close, it will try to attack you.At that time you will have a health meter intill the pokemon stops attacking.If the meter is empty because of the pokemon hitting you too much, you faint and start back at the beginning of the mission.There are several items you can find after the first mission that can help you prevent fainting.
2. One of the final missions is that you're on a hidden island and you have to take photos of all legendary pokemon except the legendary entei, suicune, and rakiou.The pokemon are in several areas on the island that suit the pokemon.(Like that groundon is in an cave near a volcano on the island, and kyogre is in an cave filled with water.But pokemon like celebi, mew, jirachi, and deoxys will be on any part of the island).
3. After beating the game, you can take photos of entei, suicune, and rakiou which will run around any area.If they sence you, they will run to a different mission area.