Actually, I was using Spellbreaker, but this was an Ancient Dragon on Master difficulty. That ward lasted less than a second. I've actually been getting lots of reports about Spellbreaker's invincibility, but it hasn't really shown that in my own usage. I wonder if it's a perk thing.
I figured that out later, and can now survive those fights by crafting potions of 102% resist fire/cold. I have no goddamn clue how that differs from a 100% potion, but that's what the numbers say.