Edit -
Ink Splitter points out that it's possible to fail the achievement by flaming the turtles and getting them into the pool that way. If you do this by accident, you'll have to quit out and restart the level.
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To reach the character who gives you this challenge, you'll first have to learn to climb, which can be unlocked in the second hub world.
Once you've got it, come back to Sunny Beach and climb up the wall that's in the same room as the ramp to the end portal. You'll meet a chef who wants to cook some turtles. You'll do two challenges, the second one longer and harder, where you have to headbutt some turtles into a pool to stop them jumping into a cauldron.
The easiest way to complete it is to stand where I do in the video, making sure you're always headbutting the turtles directly towards the pool. If one jumps in the cauldron, you'll have to restart the challenge.
You get the achievement when successfully completing the second challenge.