1.  Enter the Overgrown (Dryad) temple and go up the stairs
2. Walk around the room and pick up the glowing spots below the blue flowers in the room (Fertile Berries)
3. Go up either the left or right stairs 
4. Enter the leftmost upper path
5. Talk to the enemies walking around and they will take a fertile berry from your inventory (make sure they are positioned on the upper wall of each corridor or you will have to reset the room by leaving and coming back.)
6. They will bloat up to pushable objects that you then push into the holes to allow you to move upward. (Once placed, you can leave and come back and while the enemies may respawn, the 'stuck' one will remain in place, in case you need to go get more fertile berries.)
7. Push and walk across the enemy filled gaps until you find the weird door/pressure plate in the top left part of the room.
8. Run over the yellow flowers, or get trapped on the wall that fills you up until the 'Seeds' bar is about 3/4 full or higher.
9. Walk onto the plate and Leah should drop below.
10. Get the 'bad ending' where Leah is filled far beyond her normal size.
11. This gives you the achievement.

Note: The demon nursery doesn't give you the achievement anymore because 400 power is technically equal or GREATER than you grow in the demon nursery, whereas the Overgrown Temple is definitely larger than anything feasible in the game for Leah to grow (so far).