Edit: New information has been posted, check July 2014
The Demon Major Bosses are not much different from the Original Diety other than the colours. Compare to their Dieties, the Demon Dieties are deffenently difficult, do not underestimate their next attack.
The Demon Dieties do not have element powers like the last boss, other than their original powers, Salamander Fire, Kraken Ice. But becuase their demons, all of them have a fire element, so there is a (guessing) 10% of a fire burn. Be sure to have plenty of Serum Potions! (Find in shop, green/brown/purple)
There is no one else around you ,so no cut seen before of some random losing battle on the Creeper.
You do not see Lamia unconscious at the Chimera's nest, and you are not fighting the Kraken underwater.
I would just copy paste everything from the other guides, but that's kind cheap. So what I am going to do is re-name the original Diety guides for both bosses. All you need to know are in the original guides.
Name of Demon Dieties/Bosses:
Hell Vine Creeper
Blaze Wing Chimera
Thunder Mane Salamander
Lava Shell Kraken