Ben 10: Power Trip

After you finish the intro section (up to building the campfire), the motor home is moved out of the way and you can access more of the world. Follow the road down for a short distance and you will see a pile of rocks on the left with a purple crystal on top. Approach it and some monsters appear. Defeat them and then break the crystal to get a bonus as well as the achievement.

Note that this seems to be buggy. My son got the achievement on his first play through immediately after smashing the crystal. I've attempted it twice and it's still not popped for me :(