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