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