So this is the hardest fight if you try to engage with it as a direct fight or like normal dnd, to avoid having atleast one party member being down you typically have to engage with larianisms. Try to have most of your party having the highground when the fight starts, take advantage of things that can knock you opponent off like push or that eldritch blast invocation. Be aware of backstab and height (not rightfully) giving advantage. And if you feel extra not-dnd you can sneak with astarion and place so many barrels the entire dragon is vaporized.
