I like the system in Siralim 2 where each class of creature is strong against one class and weak against another.
However, I think this system is likely to be confusing to new players. (In particular, it took me a while to get the hang of it and I read these forums often.) In particular, there is no visual indication when this happens, and so unless you have read the appropriate book in the Library the system is pretty opaque.
Personally, I think some visual indication that this was happening (perhaps a different color of combat number, or a word accompanying the number – or something similar) would be best. Although another option would be to make a point of this relatively early (like after the boss fight on Realm 3) in NPC text.
Final note: I assume that Orange numbers are Critical Hits, although I couldn’t find a library book that told me what Orange numbers mean. (Maybe I wasn’t looking hard enough.)