Here is the full list of all 20 High Noon Revolver achievements.
Get no damage in an ambush
Get a total score of 10,000,000
Roll 100 times
x150 kill streak
Finish Act 3
Get a total score of 5,000,000
Finish Act 2
x50 kill streak
Unlock Skipp
Finish Act 1
Unlock Fara
Unlock Zero
Unlock Annie
Jump 100 times
Die.
Finish Act 4
Defeat the Final Boss
Get a total score of 50,000,000
Roll-jump 100 times
Reach the highest rank