Help with CR of BBEG

[AuraSeer]

How many PCs died in the fight?

 

[RangerWickett]

The monk got bitten and snatched and devoured over the course of a few rounds. The fighter got thrown at my sorcerer character, which wounded me a lot. The fighter ended up being okay thanks to healing spells and a half dozen "aid another" actions from allies on our side, and though the allies got hurt a lot, none died. We retreated once the monk died and the party cleric plane shifted away with the dragon's artifact. When the dragon took to the air and started to fly away, I incorrectly guessed that it's lesser globe of invulnerabiliity had run out, so I tried bolting the dragon. I mean, a dragon that lives and escapes is bad. You want to finish them before they fly away.

Well, I got eaten.

So two deaths, and a victory by theft on our part. The rest of the party fled into the city, and the dragon had to flee because an army was marching in.