Well then you are left with setting up the following...
Give the ability to resist the spell.
add the <IsForSpellResist> tag for the game-modifiers you want to keep if they did not resist the spell.
This leads to the problem that you are forced to utilize the spell-resistance stat, which is uniform and I don't think can be changed to other damage forms.
You can also force the game to not be able to target particular units, like beasts... but I don't think you can create a new creature for this targeting unfortunately.
At this point I do not know of a way to make a target immune to strategic spells and not tactical spells... If there were a way to turn off magical immunity during tactical battles I do not know it. But there is a way to make a creature more resistant to spells on the strategic vs. tactical battles, I would point you to combine some of the effects of noble-legacy and shadow bolt. This could be used to raise or lower spell resistance during tactical battles, while leaving a uniform resistance on the strategic map.
Is your purpose here to remove the use of tornado as a spell, because you can just remove the spell and substitute it with another spell in its place. Save you time.