This is on Zaleky Posad allod while leveling in Umoir, right ? The boss on a separate island ?
I don't remember any special tactics for him, your werewolf companion should do most of the job.
Ah, that quest! Had completely forgotten it. I don't think there's any tricks to it other than what Pochtron already said about the basics. Milava will heal you but after she has been saved. Don't remember did she heal you, if you were near her while killing the Demonologist.