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The Challenge Scores
The challenge scores are relatively easy to reach in most levels.
The beach quite possibly being the easiest challenge scores to beat.
By taking pictures of a few high scoring Pokemon poses, while also snapping every mon possible, the Challenge score is a sinch to beat.
(The # of Pokemon caught is multiplied by the score of the photos you've taken)
The levels that have difficult challenge scores to beat, are the River and Valley. The Valley being a bit easier with more Pokemon to take pictures of.
I find this to be more evidence of hidden Pokemon in the area.
If there were 1 or even 2 more Pokemon in the River and Valley areas, the Challenge score would be approximately as easy to get as the rest of the levels.
This being the case, because of the multiper you get for every single different type of Pokemon.
Now. We know that the River and Valley are the most likely to contain new mons. So, next, we have to find them.