Just played my first game against Sonnia with Rasputina last night. Great game! It was close, but I managed to eke out a victory.
I found that Sonnia, the Witchling Stalkers and everything else anti-casting went down pretty quick against the ranged attacks of the Golem, Gamin and Acolyte. Since these are attacks and not spells, they managed to do some decent damage. Using non-casting units in close or ranged combat against Sonnia worked well for me - she's only Df 4 and Cb 5, so she's not nearly as scary as most. I'm sure this is equally true with other units like the Convict Gunslinger, Joss, Cerberus, etc.
Oh, and I kept Rasputina the hell away from Sonnia! :peep: