Battle of the Celestial Gates

I've tried to eliminate Kane about 20 times. It's impossible. The best outcome is https://c2n.me/48hs0Hj

Kane is the last of the demons that appear. If that helps, you can try merging Kane and defending the city with another hero.

Added 12 minutes later
You can also just not worry about losing your army and lose all the creatures you have. They won't be useful anymore.