Cataclysm does not work on Rani. She resists the hell out of dark damage =/. Also, yes, you do heal it if you do a Smite > Smite Skillchain while she is casting magic.

If you don't have Victory Smite, I'd just shoot for Flash Nova or Earth Crusher with the appropriate atmas and such. Don't try to brew it with RR/GH/Apoc if you're using a Magic WS (duh). Or just use Ascetic's Fury with RR/GH/Apoc. Pretty much any WS will do as long as you Atma for it correctly and avoid healing her.