Here is the full list of all 20 Six-Guns (Windows) achievements worth 200 gamerscore.
Reach level 40.
Open 200 chests.
Kill one animal of each kind.
Kill one demon of each type.
Kill a Giant Werewolf.
Reach level 25.
Kill 1,000 enemies.
Obtain 50,000 coins.
Win 25 matches in Multiplayer mode.
Kill 500 enemies.
Use all the Road Signs in the game.
Reach level 15.
Visit Oregon.
Die once.
Kill your own horse.
Try your luck with the Lottery
Complete the tutorial mission.
Kill 10,000 enemies
Reach level 50.
Complete all quests from beginning to end.