Here is the full list of all 26 One Hit KO achievements.
Complete the tutorial
Defeat 13 enemies in one round
Defeat a total of 100 enemies
Defeat 33 enemies in one round
Defeat a total of 250 enemies
Defeat 66 enemies in one round
Defeat a total of 500 enemies
Defeat a total of 800 enemies
Defeat a total of 1100 enemies
Defeat a total of 1500 enemies
Defeat 88 enemies in one round
Defeat a total of 2000 enemies
Defeat 133 enemies in one round
Defeat a total of 2300 enemies
Defeat a total of 3000 enemies
Defeat a total of 3500 enemies
Defeat a total of 4000 enemies
Defeat a total of 4300 enemies
Defeat a total of 5000 enemies
Defeat a total of 5500 enemies
Defeat 177 enemies in one round
Defeat a total of 6000 enemies
Defeat a total of 7000 enemies
Defeat 255 enemies in one round
Defeat 256 enemies in one round
Finish the game without using the secret technique