The Frying Pan is a weapon that you get after completing 'Hidden Booty Hunt', and first requires you to collect 6 clues:
The first two can be earned after completing the missions 'Bounty Hunt' and 'Lost Trader'. The third is recieved after killing the last Assassin from the mission 'Assassin Attacks'. Number 4 is a prize from the Archery Competition in Knothole Glade. The fifth clue is in a chest in Oakvale(not sure which one) and the final one is in a chest in Windmill Hill behind the windmill.
Once all 6 clues are found, go to Orchard Farm by the barn, and use the shovel in the small patch between the haybale and the barn. Once you have the weapon, kill an enemy to gain the achievement.
You can technically get this achievement immediately, but picking up the Pan without all of the clues will mean that the Pan does 0 damage.