Here is the full list of all 28 Them's Fightin' Herds achievements.
Win 1 online match.
Win 10 online matches.
Win 25 online matches.
Play 1 online match.
Play 25 online matches.
Complete Arcade mode with Arizona on Greenhorn or higher difficulty.
Complete Arcade mode with Velvet on Greenhorn or higher difficulty.
Complete Arcade mode with Oleander on Greenhorn or higher difficulty.
Complete Arcade mode with Paprika on Greenhorn or higher difficulty.
Complete Arcade mode with Pom on Greenhorn or higher difficulty.
Complete Arcade mode with Tianhuo on Greenhorn or higher difficulty.
Complete Arcade mode with Shanty on Greenhorn or higher difficulty.
Complete Arcade mode with all "starting 6" characters on Greenhorn or higher difficulty.
Begin Story Mode.
Complete the Story Mode prologue.
Complete Arizona's Story Mode chapter.
Obtain 100% of collectibles in Arizona's Story Mode chapter.
Stomp the Son of Deer painting to pop out a hat.
Stomp Paprika into orbit.
Complete all combo trials for Arizona.
Complete all combo trials for Velvet.
Complete all combo trials for Oleander.
Complete all combo trials for Paprika.
Complete all combo trials for Pom.
Complete all combo trials for Tianhuo.
Complete all combo trials for Shanty.
Complete all combo trials for every "starting 6" character.
Unlock all other achievements.