I missed a Mana Egg in the previous dungeon, but it's easy to go back and get. Re-enter the forest and go northwest, over a large flower trap.
Fight your way through a few wimpy enemies and take the right fork to a dead end with a large flower, and there it is.
I also took the opportunity to upgrade my equipment and use my two Mana Eggs off-camera. Justin and Feena now have access to all four elements.
