You do realize that this is just her
Aspect yes?
God's don't have discorporation - their Aspects do, meaning that when the god's aspect is defeated - the essence of the god returns to its home plane to recover. To truly kill Tiamat, you would need to travel to her fortress in Avernus, and confront her there.... That would make for an epic storyline to be sure.
Were I to run such a campaign - I would alter Tiamat's stats as follows:
Tiamat (Avernus)
- Increase her CR to 35.
- Remove Discorporation (only applies to aspects)
- Increase HPs to 999
- Keep all of her resistance/immunities from her aspect.
- Grant her spellcasting as a 9th level Cleric and 9th level Wizard, (aka INT + 20 arcane spells, and WIS + 20 divine spells).
- Grant her the ability to use quicken Spell, twin spell and heighten spell as a sorcerer, and 20 Spell points. This will give her a much needed bonus action.
- Grant her the following feats: Alert, Elemental Adept
- Grant her Lair Actions.
If slain in Avernus, Tiamat herself has been slain. Ressurecting a god is a monumental task, and requires magic beyond that of a 9th level
Wish or
Miracle spell.