Here is the full list of all 12 WESTSLINGERS achievements.
Complete the campaign on any difficulty
Complete the campaign on Most Wanted with permadeath
Complete a mission with an outlaw in critical health
Steal the money lost by a banker
Complete a mission without killing anyone
Earn a nickname
Blind 4 victims with Pocket Sand all at once
Go on a 5 kill spree
Go on a 10 kill spree
Kill an enemy at the max visible range
Kill 2 enemies with one shotgun spray
Get a ricochet knife kill