One thing worth pointing out about the spell..
Tornado weakens a target between 1 and 9 HP.
If for any reason the current HP of the target is greater than or equal to the randomized value Tornado generates, the spell will fail to work.
This is why the spell sucks vs the Imps/Goblins. Imps/Goblins have 6 HP, so there's a 4/9 chance of failure.