Yeah, Monster Math is kinda an impenetrable mess. I would say that raw attack and defense are the priority, but generally I try to match up weapon elements with monster weaknesses as best I can. As a GS guy raw matters way more for me for the reasons @jsnell outlined.
Armor is a slightly different matter. While I don’t give up raw defense I definitely eat for the elemental resistances the set I am using is weak in if I need to. For example, fighting a Rath with armor weak to fire is going to be pretty rough unless you offset that weakness somehow.