Stupid...just plain old stupid!
It looks like I will never be able to solve Worst Nightmare Rank 6. It requires a vent takedown which can only be performed BEFORE YOU BEAT THE GAME. Now I feel like breaking this gd game in half for all of its stupidity.
First I tried on the Penguin's ship, but found out that floor grate takedowns are not vent takedowns. Then I scratched my head wondering where the f am I going to find a vent, take the cover off, and do a takedown through another vent. You can't do it in any of the Predator rooms inside the buildings!
Arkham City, I believe, intentionally made one of the Predator rooms (the subway) so that the enemies would come back every time. That allowed you to complete the challenges. It looks like Origins failed to do this.
I am so mad at this game...and WB.