Here is the full list of all 43 Only One achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Skeleton 2 Dead
Go Left
Go Right
First Restart
1 Lvl completed
Music 1
Start Game
2 Lvl completed
Music 2
3 Lvl completed
Music 3
4 Lvl completed
Music 4
1/3 of the game
5 Lvl completed
Music 5
6 Lvl completed
Music 6
Middle Game
7 Lvl completed
Music 7
Bandit 2 Dead
8 Lvl completed
Music 8
9 Lvl completed
Music 9
Bandit 1 Dead
10 Lvl completed
Music 10
Pause
Pause 2
Skeleton 1 Dead
11 Lvl completed
Music 11
Game Over
First Death
12 Lvl completed
End Game
Jump
Fall
Last Step
Mute
Music 12