"I realized that frost doesn't work on machines during my adventures in a dwarven ruin..." -Blisterwort-
I was making my way through Mzinchaleft with Blisterwort and at the end there was this Dwarven Centurion. Blisterwort specialize herself in frost magic and I thought it would slow it down, making it more easy to kill. Boy, I was wrong.
Blasted machines was immune against frost magic!
Blisterwort made it out alive with just a weak firespell and tons of health potion.(after 13 hours of horror and pain) Still a little shaken though.
Personally, I found that summoning worked wonders with centurions. I once let my flame atronachs go at it, and the thing just puttered around while it was being pelted with fireballs. It's like they can't recognize magic constructs, or something. Go figure.