To get this achievement, you need to play all the way through the level "Walk In The Park." Once you get to the end you will see Shazza standing next to her jeep. Go up to her and click "Y" to interact with her. She will ask you to defend her while she fills up her canteen and will put you in a mini-game with a bunch of boulders in front of you. Simply kill all 10 of the blue tongues without missing with a single boulder. Timing is key on this one. If you fail or have already completed the level, don't worry as you can come back and do this again.