Tiamat and fire damage
So if I were to be fighting tiamat, and I true polymorphed into another tiamat, would I be able to use fire breath against her, or would we need to do melee combat only? And would our claws be able to do anything to each other or would we have a stalemate?
(assuming 5e or newer) You can't true polymorph into Tiamat. The 'aspect of Tiamat' from Fizban's is already CR 30, Tiamat herself is unstatted but obviously more powerful.
As a player character, you can only True Polymorph to a creature of CR 20 or lower