Here is the full list of all 19 One Gun Guy achievements.
Beat the game on normal difficulty.
Beat the game on hard mode.
Beat the game without dying on normal difficulty.
Beat the game without dying on hard mode.
Beat the game without taking damage on normal mode.
Power up to invincibility.
Collect 50 coins.
Collect 150 coins.
Collect 250 coins.
Collect 500 coins.
You found a hidden area!