Huh? I've not played much QoT, but I have played some Rook and against the AI I can easily farm kills. In fact, against the AI you can actually kill something with Hammer Slam.
sure, the problem is not that you won't get kills. You'll probably get 15 kills or something, the problem is that if you play a EB, UB or reg for example, you'll easily double that.
It's like when you play the tournament singleplayer on nightmare, and it goes into slaugher. It is actually very hard as Rook.