You will unlock this shortly after entering Agata Forest.
You need to go down to the beach in Agata Forest (basically go the only place where the cursed area isn't).
You'll see a cave. Go through, and make sure to save your game at the mirror to avoid missing this achievement due to achievement glitches.
Talk to the woman inside, and get the Stray Bead from the chest. Then, leave the house, and the cave. Instead of going up the stairs away from the beach, follow the path around the outside of the cursed water to another cave.
Paint a Cherry Bomb at the end, and go through. Heal the Sakuya Sapling with Bloom, and when you leave this cave area, you'll meet Waka.
Fight him, defeat him, go through the cutscenes, and when you regain control, the achievement should pop.
If the achievement doesn't pop, reload your save and try again.