Firstly, I think it would be good if the Necronomicon was a bit more explicit when it comes to monster types. For example, it's not made clear that Shoggoths are in fact undead, and therefore affected by Smite. I bet many people who complain about their "high HP" don't know about this. Also, the Shoggoths in the Dark World are apparently "Dark" type and are affected by Light Pierce instead.

Secondly, I think some enemies could have their types changed.

Dreadguard: Change from Undefined to Undead.
It walks like a zombie, sounds like a zomibe, yet it is appearantly not affected by Smite.

Remnant, Gatekeeper Minion and J'Zahar: Change from Undead to Shadow.
Right now the Light Pierce enchant only affects Shadow Creatures, which is pretty niche.

Change Lesser Dreadbeast and Spawn of Cha'Garoth from Undead to Arthropod.
Almost every mob in Abyssalcraft is undead, changing these two to Arthropods adds some nice variation to enemy types.

It's also worth noting that Cha'Garoth is currently the only boss without weakness. To be fair, I have no clue what to classify him as.