To unlock this Achievement, knock one of the cow-like creatures into the lava inside the volcano.
You can either use your vehicle to bump and bash the cow across the map until you get it into the volcano, or you can pick it up with the wrench using the R-trigger, which is much more time-consuming. You must place the cow into the bright yellow lava pit in the center of the volcano - it won't suffice to put it in the red flowing lava on the outside.