Well, on the topic of invincibility, both Nightmare and DM are smart enough to cover their weakpoints AND discard their only weaknesses. Also, I realized that because of Mirror World physics, they can't be fully killed. You can put one down for a while, but the other will become the true embodiment of fear, and because of Mirror World physics, the recently-killed will just get up again and wait for the other to be killed so he can return to being the embodiment of fear. ALSO. DM's ability to take control of people's brains is an issue.