To earn the achievement that's called "Is That A Quarter?", you have to knock Williams into a pit.
This is easily done in Stage 2, which is called Back Alleys
Near the end of the stage, a little before you enter the rocket ship, you'll come across a huge pit. On the left side is Williams, looking downward, saying something like "Wonder if there's anything down there?" or something like that. All you have to do is get behind Williams, press Y, and watch as Billy or Jimmy kick him from behind. He'll fly into the air and fall into the pit, netting you the achievement.
Have fun and good luck!