This achievement is: "Give Me My Gold Back "
Defeat Mayor Hamm in the Woody's Roundup boss battle
Note: Make sure you've done the mission to take a picture of the Mayor at the town hall (push him there with a car) before doing this or you might miss the Photojournalist achievement as i haven't seen the mayor since doing this. ARCHER MIKE has commented that you won't miss the Photojournalist achievement if you don't get a picture of the mayor.
You go into this boss fight immediated after completing the Legendary Treasure mission, which is completed by jumping into the well while carrying a TNT barrel and blowing up the blocked passage to the treasure.
You'll be back up in the town. Keep picking up TNT barrels and wait for Evil Dr. Porkchop to activator the money grabbing tractor beam. Throw the barrel into the beam when it activates. It will take a few barrels to beat the boss.