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
Oh, well thanks for clarifying that. In that case, assuming both are accompanied by a party of a level 20 rogue with fudged stats and a ton of proficiencies and expertises, a level 20 druid with normal stats, and a level 20 barbarian, who would you give a better shot at defeating tiamat?:
a level 20 bard with unlimited magical secrets(including wish), or: