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Mallet of Thunderous Skins
Craft the Mallet of Thunderous Skins, the Tool of the Trade for Leatherworking.
The 'Mallet of Thunderous Skins' achievement is earned by crafting the 
Mallet of Thunderous Skins, the Battle for Azeroth Leatherworking Tool of the Trade, which requires completing a questline to unlock the pattern and reaching 150 skill points in Kul Tiran or Zandalari Leatherworking.
Complete the Questline
Complete the Leatherworking Tool of the Trade questline for your faction (Horde or Alliance) to unlock the crafting pattern for the Mallet of Thunderous Skins. You must also have 150 skill points in Kul Tiran or Zandalari Leatherworking before you can craft it.
Farm Long Regal Sinew
Collect 4x 
Long Regal Sinew by entering Atal'Dazar and defeating Rezan — reset the dungeon and repeat until you have all 4.
Farm Spirit of the Bested
Collect 15x 
Spirit of the Bested from Avatar of Xolotal; only one can drop every 15 minutes of server time, so kill the boss once every 15 minutes and this will take a minimum of approximately 3.5 hours of real time across multiple kills.
Craft the Mallet
Once all reagents are gathered, craft the Mallet of Thunderous Skins at a Leatherworking table to complete the achievement.
Tips
Spirit of the Bested follows a server-wide 15-minute timer — kills within the same 15-minute window will not yield a second drop, so space your kills exactly 15 minutes apart to avoid wasted attempts.
The Profession Shopping List addon can track the required reagents and calculate how many you still need by using /psl in-game.