How do I get the True/Best ending?

To get the Best ending it seems like you need to view certain cutscenes in Hidden Dungeons (found by scouts). How many of these cutscenes are there and where? Is there anything else I need to do?

In addition I think you need to get some key items to

Rescue Peashy, netting the "good" ending

But I'm already aware of how to do that. So if I've already unlocked the Good ending, what extra do I need to do to ensure the Best ending?