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Planta fought a dangerous opponent with overpowered, cheat-like abilities by abusing her own overpowered cheat-like abilities, it was as simple as that.
When Merlinus was surprised to see her appear from underground, it was because she had set up her return in that moment her first body was destroyed or sent away.
By discarding her first body completely, she reanimated a Seed she had planted underground, one of many, into a new Yggdrasil Avatar in a matter of seconds.
These Seeds were easy to plant, her very steps expanded her Domain and Terrain, and seeds would spread everywhere!
She discovered that she could select where she wanted her Avatar to be born, and it didn't have to be from the wood of her true body, the giant tree in the Forest of Beginnings.
As long as it came from her, it could become a new Avatar, and Seeds became the answer, she spread them all over the field, although the amount of Avatars she could make was limited.