That's good, but now it will take some time drop the turtle down if you're on impossible... Those 4x1 archmagi will take quite some time to do it, unless you can back them up with spells and magic spring to replenish mana
Still, this is imaginative. Good work.
EDIT: As for the royal thorns tactic, it works if you have a decent spell arsenal and magic spring to replenish mana. Get Hagrid (heals/resurrects thorns - his description is misleading), get royal thorns, keep the royals defending with magic spring on and just bash the spider with spells. I did that with a Paladin + Magic Pole Axe (took ages!), so a mage would have much easier time.