I am at work so I can't pull up my game. But I had my summon demon skill maxed, summon mastery maxed, health leech maxed, health regen maxed, and plenty points in mana leech, sword and shield mastery, heal, and bleeding. The game is tough, but the build did me well. Even in the arena battle vs all the previous bosses, I was just able to click away without dodging or casting any spells aside from summons, since I was generating health with every hit. I felt as though I must be well prepared to take on the final battle. I don't remember my exact equipment, though I had on some--but not all--of a set; scorpion set maybe? And a nice, charmed-up, enchanted-up warhammer.
But The Divine and Zandalor heal faster than I can damage. If I separate them so that me and my summons can wail on just one, I can hammer for a long time and get nowhere, because as soon as he gets to just under 50% health, he heals. I could finish on easy, since my melee creature had equipped the insta-death skill that lets him kill an enemy under a certain percentage of health, but on medium it was literally impossible for me. I guess in the end it doesn't matter; after hammering on the same two dudes for an hour, I switched the difficulty and was able to get through. But I was--and am--frustrated with the issue, which imo stems from some clear flaws in the very fabric of this game.