Hey if it's taking you that long to kill him, consider powering up a Blood economy of your own on the side - very cost effective once it's started. Then you can get your own Fiends of Darkness and whatnots, really fun stuff.
Though Earth Elementals and Umbrals are pretty nice already.
Did you try Magma Babies on him at all? Many demons are actually fire vulnerable, and those Babies have insane damage output.