The party is looking at facing Tiamat as the 20th-level epic boss fight that will end the campaign. The evoker has only four spell slots that can directly affect her, due to her limited magic immunity, so has to choose well.
He is considering Arcane Hand, which when upcast has good damage output, considering her immunity to most of his damage types and her legendary saves. However, the Hand, as an object, automatically fails Dex saves and so will go down pretty quickly to fire, lightning, and acid breath. The Hand, as an object, is immune to poison.
I was confused about her cold breath. White dragon cold breath is a Con save; Con is a pretty standard save against cold damage (cf. Cone of Cold, Freezing Sphere). As an object, the Hand would also be immune to damage that came with a Con save. But the statblock I have for Tiamat in Roll20 says that her white head's cold breath causes a Dex save. I don't have access to the print copy of Rise of Tiamat.
Is the Roll20 cold breath correct, or in error? If her printed statblock actually does call for a Dex save, is there any precedent for that - perhaps in previous editions, or any official reason why it would be different from white dragon breath?
Matswecja's answer resolved my wizard's decision by pointing out that dragon breath, including Tiamat's, as written can damage only creatures, not objects. However, it does not explain why Tiamat's cold breath works differently from a white dragon's. I am still seeking an explanation for this - it could be a legacy of her stats in previous editions, a deliberate change signaled somewhere, or simply an error in editing.