I was thinking more along the lines of making the mirrors into a bomb. Steal a bunch of them, then shatter them simultaneously at the tower. You'd need a lot of mirrors, but an average deity has only 400 hp. Small mirrors do 5d6, medium do 10d6. Average damage, fort save for half, so each small mirror will reliably give around 9 points of damage, and each large mirror gives around 17. You'd need 24 medium mirrors to ensure 410 damage. But you only have to be five miles away from the fortress! And since each mirror destroyed harms Izzy farther (I have no idea by how much), and most of the fortress is going to be destroyed as well, that's quite a telling blow.
Of course, the valient heros are going to die in the process. And getting that many mirrors will be difficult (to say the least).