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
Skeleton 1 Dead
12 Lvl completed
End Game
Last Step
Music 12
11 Lvl completed
Music 11
10 Lvl completed
Music 10
9 Lvl completed
Music 9
8 Lvl completed
Music 8
7 Lvl completed
Music 7
6 Lvl completed
Music 6
Middle Game
5 Lvl completed
Music 5
4 Lvl completed
Mute
First Restart
Game Over
First Death
Music 4
1/3 of the game
3 Lvl completed
Music 3
2 Lvl completed
Music 2
Bandit 2 Dead
Bandit 1 Dead
Go Left
Pause 2
Pause
1 Lvl completed
Jump
Go Right
Music 1
Fall
Start Game