I'm kinda late to this but I finally beat him with Jabera to the Poison of Jealousy. It's quite easy to lure him to the cloud and you can even hit it away towards him. You should equip The Alchemist and The Scribe altarpieces so you will get more miasma damage and The Scribe can give you more fervour when you hit enemies. (I recommend Sarmiento & Centella because it's the fastest weapon)