Y'all are talking about order of casting between Invulnerability and skin spells, but I don't think the game cares about that, except when it's running the spell-casting AI. My testing suggests that it all works in this simple fashion:
You can have one of four kinds of "skin" (where ordinary, non-magical skin is one kind). You can have one of two "invulnerability" states. Skin state is independent of invulnerability state. Since only one skin can be present, only one skin-related elemental vulnerability can be present. When tougher skin is applied, it replaces weaker skin completely. When weaker skin is applied to tougher skin, nothing happens. Again, this paragraph is about spell effects, not about the spell-casting AI.
The spell-casting AI seems to assume that if a "skin" or Invulnerability spell won't increase protection, then it shouldn't be cast. But if Wooden Warriors hits somebody with no magical skin and also somebody with ironskin, the ironskin, as stated above, will not change.
Invulnerability's protection-increasing effect seems to override the protection provided by a skin.
Strength of Gaia causes a barksin effect and some other effects. The barkskin seems to be independent of the other effects.
I know some of this has been said, but I think not quite all of it... and surely you understand the impulse to pedantically explain things, fellow wargame forum reader.
