When looking to unlock the Mage opponent, you have to turn in one of each amulets in the same go including the two newer ones. It's not retroactive. So you have to turn in all 10 amulets (not including the rainbow of course) to unlock him.

If you just got the achievement for the Decidiwi and did not get the branch, you can file a ticket and have them manually award it to you. I would do an account search beforehand to make sure you didn't stash it somewhere.

Hi - I know it's been ages, but I wanted to follow-up to make sure this was resolved for you. Did the above post by Destiny help? Or are you still unable to unlock him?