Here is the full list of all 25 Western Press achievements. It takes around 2-3 hours to unlock all of the achievements on Windows.
Win 100 duels
Win 50 duels
Win 25 duels
Hide a character in the spittoon
Subscribe to a Workshop character
Subscribe to a Workshop environment
Win a match of Memory Mode
Win a 16 competitor tournament
Don't get shot in a tournament with 8 or more competitors
Win 10 duels
Unlock Chief
Watch the epilogue
Beat level 9 of the Skill Tester
Beat level 8 of the Skill Tester
Beat level 7 of the Skill Tester
Beat level 6 of the Skill Tester
Beat level 5 of the Skill Tester
Beat level 4 of the Skill Tester
Beat level 3 of the Skill Tester
Beat level 2 of the Skill Tester
Beat level 1 of the Skill Tester
Play an online match
Remember a 20 button combo in Memory Mode
Remember a 15 button combo in Memory Mode
Remember a 10 button combo in Memory Mode