Here is the full list of all 20 Boot Hill Blaster achievements.
Finish a boss level without taking damage.
Clear level 25.
Score 1000 points in a game.
Clear level 20.
Kill 10 enemies with one shot.
Score 500 points in a game.
Clear level 15.
Score 250 points in a game.
Score 100 points in a game.
Clear level 10.
Clear level 5.
Score 50 points in a game.
Grab the time freeze.
Grab the rocket.
Grab the horseshoe.
Grab health.
Kill 5 enemies with one shot.
Finish a level with only 1 health remaining.
Kill 2 enemies with one shot.
Grab the all-way.