So, we're playing mission 3 in the main set (PRoblems of Life and Death) and the one Naga at the end apparently can't be hurt by nromal attacks.... but there is nothing defining what is and is not a normal attack. Little help?

Yes, it means that the Naga cannot be hurt by regular Hero attacks. Read the complete text for Area 5 in the quest and it tells you what the heroes have to do to defeat the Naga.