This Achievement is gained by completing the Finals of the Colosseum Kickball Tournament in Hailfire Peaks.
In order to access this tournament, you must have completed the finals in the Mayahem Temple Cickball arena. This ensures that you know how to play the game, and that you have access to becoming a Stony.
There is a wall with a large crack in it to the right side of the Stony on the far side of the area when you enter. Use a grenade egg to destroy it and open a door to the Mayahem Temple Colosseum area. Go to Humba Wumba and transform into Stony Banjo, then return via the portal to Hailfire Peaks. You can now access the Quarter Finals.
All the matches are timed, but you should have no problem gaining the lead fast and keeping it.
The Quarter Finals require you to kick as many yellow balls into your opponent's nets as possible. As long as your score is the lowest on the board, you will win. You can have anywhere from 0-100 goals, as long as the next lowest score is at least one point higher.
The Semi Finals are a bit more difficult. Each player starts with 50 points, and you only have access to red balls. Get as many into your own net as possible to lower your score to move on.
The Finals show a mix of yellow and red balls, as well as bombs. This is just like the Finals from Mayahem Temple, except you want to score as low as possible.
Remember that Banjo, though a Stony, still moves faster than the other contestants, so even if they are closer to any given ball, you can probably still beat them to it. And don't forget your tackle attack - it's a boost of speed when you need it most.
You get a Jiggy for completing the Finals, as well as this Achievement.