Need Help with "Rare Item Wanted"

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I'm trying to do Isau's quest in Snakemouth Den and I'm not sure how to get to the top of Snakemouth. I've noticed that there is a waterfall similar to the one in the metal lake island. Is there a powerup for Lief that I'm missing? I'm at the final boss in terms of the story so I don't think I missed a powerup. I'm trying to do every quest beforehand.

[wataniyob]

Go to the room with the first big door in Snakemouth (the room where you fell down a trapdoor). Take the spring up to platform where you originally knocked down one of the rocks used to press down the buttons. Then use Vi's ability to fly to the left (resting on the door top) to find an exit from the top left of that room.

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Thank you. Here i was thinking I missed out on some ability.