Clearly sub-Chadding it with Blasphemous Blade.
I tried again last night and somehow ninja-warrior'd my way through through all of the fire, lighting, tactical nukes, and fucking laser beams and chopped him down to one or two hits to go with plenty of health flasks left. He was getting ready to throw out his fire breath, so I was all...cool, imma just roll out the way and run back in and fuck him in the dragon ass, but nope. He had slightly shifted over and somehow I was stuck under his belly so couldn't walk, run, or roll.
Flames. Scream. Dead. FUCK.
This is one of the few fights where the mimic dies well before me. Even if I wait until the second phase to drop him out, he walks right into every attack and just gets vaporized in like a minute. Maybe it's time to try another weapon? I've got a healthy collection of +10 stones.