That's the problem, to finish the game... xD
Secret Herbs
Loot=Rubin Elf, Neryil Soldier (both unaviable)
Collect=Yuidora Woodland (I guess this is the only way).
I'm gonna try randomly in Yuidora Woodland for now, thnx for the help!
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EDIT:
Confirmed. Secret Herbs are found in Yuidora Woodland high lvl areas, just by harvesting.