Heroist
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The Chaos Dragon has a damage formula.
In theory, you could also try to stun it, if that works on the dragon. Then, with Thaelina focusing on one-shot attacks (even with a 20% chance over 12 turns, it's a guarantee if it works), along with Iwamoto, Randell, and Telania, but I don't know if it will work.
Or just try to max out Bloodshot and fly to a town if you can't hit it. :D
And do that every day until you succeed.
Whatever