"Don't let them cast it at all" is the best approach
Fire Tooth +3 is a WONDERFUL mage disruption tool. Spell Immunity VS. [whatever type spell Insects are, abjuration?], and I think all 9th level spells can't be disrupted oven if you take damage. I think magic resistance also affects this because no dragons I fought ever suffered this. Luckily, you only have to go head to head with this spell a couple of times in the Druid Grove. Try Spell Shield as well, though I havn't tried this.