For some reason, I can't achieve the bully achievement (kill a Demigod 10 levels below you).

I just played with the Rook, I was level 18 or so at the time and attacking a level 7 frost/fire Demigod. I killed him, got the kill, but no achievement. I thought ok, maybe the towers attacking him on my shoulders killed him and maybe that doesn't give the achievement. So, I killed him a few other times while he was still under level 10 and I was level 18-20.

I then thought maybe it just gives it to you once you finish the game.

So i just went ahead and smashed them, once I was done I looked at the achievements for the rook and I still didn't get it.

Am I missing something...?