I managed to do this with a Celestial Nighthawk Golden Gun and the new Raid Armor mods. 20% bonus damage from the Gauntlets, another 25% from the Class Item. I think Empowered Rift also amplifies Golden Gun damage. It was a critical hit, and we had 104 stacks of the Force of Will buff.
It took out a big chunk out of Emperor Calus's health bar (about 1/6), it was hilarious to look at.
I really like some of the new Raid Armor mods, I wish they would work outside the Raid. The helmet perk is kind of lackluster though.
Have you tried them yet? Your opinions?
EDIT: picture link in case the above image doesn't work: https://i.imgur.com/sFVZtkS.jpg
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If you run bottom tree celestial nighthawk on the shattered throne, when you get to the final boss, if you damage all three knights and shoot the boss you can also do 999,999 damage