Have someone with at least two levels of Sorcerer, get that Twinned Spell metamagic, and cast True Polymorph. On one of them (at least), turn into Bael from Mordenkainen's Tome of Foes.
That gives you a couple of useful abilities. Dreadful means that you can force Tiamat into a WIS saving throw to make her frightened once per turn for free, and you can also force her to burn through her Legendary Resistances (or take the loss of advantage against spells). Once those are gone, you've also got a cast of Dominate Monster, which -- if it lands -- ends the fight then and there. Order it not to resist as you turn it into something small and squishable, then cast Flesh to Stone and grind your new statuette into a fine powder.
With the Twinned Spell, you have two Baels to try this out with, and then you've still got all of your abilities (minus one Level 9 spell) when you're done.