At least 3 of the Omen achievements are mixed up. I have attained them, but their descriptions/how they are unlocked are mixed up. I got the Demonic Marksman when I landed 500 Rashakukens for example and got Demonic Annihilation when I did a combo that did over 85%.
Also:
http://doomsaying.ca/KI/AchievementError.png
This description is probably a mistake?