No, Invulnerable will make it immune to magic too. He wants it immune to non-magic attacks.
Hardened skin should be enough. Modify it so the minimum damage is 0 and the max damage is some ridiculously large number. Unless hardened skin also reduces magic damage.
If it does, then try changing the unit's armor type to divine. Oh, and edit the divine armor type in the gameplay constants to allow spell damage (set it to 1.00 I think).