How do I defeat the divine?

Focus on defeating the weaker enemies, then focus on the Divine. Don't engage him with melee or close up attacks. Simply run away from him and attack him from a far with range weapons and magic. If you have firewall, healing, magic missles, summon demon, summon ghost, summon undead, magic blast and/or fireball use them.

Just remember to be patient and to stay away from him.

I had level ten fireball, summon demon, a powerfull creature, mostly fresh and above quality pieces, lots of potions, the red ones and the silver ones, you can use both at the same time. I use fireball to kill all weaker enemies, attack Zandalor, then kill Lucien, when you need to heal profusely take a strong red potion, as well as the silvery one, if needbe, then jump away to regroup, you can kill him with melee if you're smart about it, it works the majority of the time for me, unless I'm in way over my head.