Dark Wave... yeah, I don't see it as dark elemental. One of the reasons that it is scary in-game is that it is completely unstoppable, unlike, say, A-Tur's similar yet elemental Frost Breath. No way do I see Vivi flashing some FF9 equip nonsense and nulling it. (Normally this is a good thing for Fou-Lu, but yeah, it means I don't see him hitting weakness on Mewtwo.)
FE7 tomes actually do have "elements", just in the case of the Thunder tome, that element is anima. Granted, it makes more sense to split Anima into Fire, Thunder, Ice, and Wind when considering it against other casts.
Other than that I don't really hold with the assignment of complete plot elements such as Dark Wave, especially since "Dark" in this case probably refers to the fact that Tyrant is a "dark" dragon in BoF tradition, rather than the associated element.