Hello there
I've been play with Erebor vs Mordor with the AI, have been 2 times that The Witch King has been killed by Thorin III when he use dwarven grudge ability (probably) and even when WK has his entirely health. I've been noticed just with this hero, not sure with more, but he can do an incredibly massive and deadly attack on the witch king with just one hit/power.
Thank you.