Here is the full list of all 32 Oniken achievements.
Find the secret on mission 2
Kill 100 enemies.
Cleared Mission 1 without dying
Cleared Mission 1
Kill 50 enemies.
Find the secret on mission 1
Kill your first enemy.
Kill 500 enemies.
Cleared Mission 2
Cleared Mission 3
Cleared Mission 4
Cleared Mission 5
Cleared Mission 6
Cleared Mission 2 without dying
Cleared Mission 3 without dying
Cleared Mission 4 without dying
Cleared Mission 5 without dying
Cleared Mission 6 without dying
Find the secret on mission 3
Find the secret on mission 4
Find the secret on mission 5
Cleared Mission 1 in less than 2:30
Cleared Mission 2 in less than 3:00
Cleared Mission 3 in less than 2:30
Cleared Mission 4 in less than 3:40
Cleared Mission 5 in less than 5:00
Cleared Mission 6 in less than 5:10
Cleared the Boss Rush mode
Cleared the Hardcore Mode
Rescued all prisoners in Special Mission 7
Kill Hackan on Mission 5 only with your sword
Find the secret on extra mission