I am so annoyed by that Drusilla. She keeps passing, and at the end, she casts a Firezilla or a Heckhound on my wizard. I have fire shields, thermic shields and all, but its really annoying when she destroys all of them. And, what's more, my wizard is a pyromancer, so you can imagine the difficulties in defeating your own element. I tried converts, and they work, but when I cast the second convert or so, she will cast a Ice shield.
When fighting the same school as yours, best thing to do is to arm yourself with a wand of the opposite school or a few 1 pip spells, TC bought from the bazaar or library. In yor case that would be frost beetles from the Library in Wizard City. They cost only one pip. Get your prisms in place early. The shield only works if it's cast before the prism. So put the prisms up, then use an ice wand or frost beetle TC to clear any ice shields she put up while you were setting prisms. Then when your attacks hit, the prisms are already there and even she she adds more ice shields, they have no effect. Hope that helps.