Personally, I think it's best if enemies who also have a story role and a lot of appearances (Dracula and Death) should have a seperate enemy data sub-page, but I think it's not neccessary to do this for most enemies or bosses. My reasoning is that the most informative/interesting information is the enemy data, so splitting the page results in the main article looking a bit barren with few pictures or info.
Example of enemy data within the article: Succubus
Example of enemy data being split: Dullahan / Dullahan/Enemy Data