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