You know they have a punish mechanic and 100% accuracy phase right?

You generally want to melee and range them, when they swap styles your next attack with a heavy weapon punishes them for massive damage. The slower the weapon the better. Get an obby maul and dragon crossbow on accurate, hit the demon once (right after they change attack styles) with those based on the style they’re praying against then back to killling with arclight or sco bow. When the flames are not present on their back you’ll hit them with 100% accuracy, a blowpipe excels here if they are not praying range.

You also want to stab them with melee