36 Bigger the stronger, how to be stronger?

Demons.

There were hundreds of different kinds of demons out there.

There were Dungeon Demons, Dream Demons, Flame Demons, and a bunch more. The variety was nearly innumerable. However, these demons could be further categorized into different subcategories.

Elemental Demons, Physical Demons, Mental Demons, and Unique Demons. 

Elemental Demons had elemental demonic tricks. Fire, Water, Wind, Earth, and all of the other similar abilities. If a demon had one of those, they were most likely an Elemental Demon. 

Dona was an Elemental Demon with the Flame Bloodline, for example.

Physical Demons were Demons with Physical Abilities. Shapeshifting, Gigantification, or similar abilities would be counted under this category.

Then, there were purely Mental Demons. Succubus, Incubus, or Wrath Demons. Such creatures would all be categorized under this category.