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I did it in the abandoned temple with the corrupted seeds. If you go far enough in on one of the earlier levels (can't remember which), there's a pressure plate. Waiting until Leah is big enough to trigger it will lead to a cutscene that will complete the achievement.

Hi! This is caused by a weird character in one of the files.
In the game folder, you should see a folder "www".
Within that folder is a folder called "data".
Find the file "Map364.json" and open it in Notepad (or Notepad++, if you have it).
In the 11th line of the file, way over to the right, you'll find a snippet of a line of dialogue beginning with "you helped me".
The "'s" in "it's" in that snippet is what causes the error.
Replace it with the one from the "let's" in the 6th line of the file and save the file and that should fix the error.
It worked for me, at least.